Add checks to all the VFS paths for "are we in a RO snapshot?".

Note - we don't check this when setting inode options via our xattr
interface, since those generally only affect data placement, not
contents of data.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reported-by: "Carl E. Thompson" <list-bcachefs@carlthompson.net>

Add a new superblock section that contains a list of
  { minor version, recovery passes, errors_to_fix }

that is - a list of recovery passes that must be run when downgrading
past a given version, and a list of errors to silently fix.

The upcoming disk accounting rewrite is not going to be fully
compatible: we're going to have to regenerate accounting both when
upgrading to the new version, and also from downgrading from the new
version, since the new method of doing disk space accounting is a
completely different architecture based on deltas, and synchronizing
them for every jounal entry write to maintain compatibility is going to
be too expensive and impractical.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Add two new superblock fields. Since the main section of the superblock
is now fully, we have to add a new variable length section for them -
bch_sb_field_ext.

 - recovery_passes_requried: recovery passes that must be run on the
   next mount
 - errors_silent: errors that will be silently fixed

These are to improve upgrading and dwongrading: these fields won't be
cleared until after recovery successfully completes, so there won't be
any issues with crashing partway through an upgrade or a downgrade.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

The next patch will start to refer to recovery passes from the
superblock; naturally, we now need identifiers that don't change, since
the existing enum is in the order in which they are run and is not
fixed.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

similar to prt_bitflags(), but for ulong arrays

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

we need BCH_SB_ERR_MAX in bcachefs.h

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

BCH_REPLICAS_MAX isn't the actual maximum number of pointers in an
extent, it's the maximum number of dirty pointers.

We don't have a real restriction on the number of cached pointers, and
we don't want a fixed size array here anyways - so switch to
DARRAY_PREALLOCATED().

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reported-and-tested-by: Daniel J Blueman <daniel@quora.org>

Add support to darray for preallocating some number of elements.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

We sometimes use darrays for quite large buffers - the btree write
buffer in particular needs large buffers, since it must be sized to hold
all the write buffer keys outstanding in the journal.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Move the slowpath (actually growing the darray) to an out-of-line
function; also, add some helpers for the upcoming btree write buffer
rewrite.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>