Minor fixes.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

For allocating XArrays, it makes sense to distinguish beteen erasing an
entry and storing NULL.  Storing NULL keeps the index allocated with a
NULL pointer associated with it while xa_erase() frees the index.  Some
existing IDR users rely on this ability.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

These convenience wrappers disable interrupts while taking the spinlock.
A number of drivers would otherwise have to open-code these functions.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

The xa_reserve() function was a little unusual in that it attempted to
be callable for all kinds of locking scenarios.  Make it look like the
other APIs with __xa_reserve, xa_reserve_bh and xa_reserve_irq variants.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

This version of xa_store_range() really only supports load and store.
Our only user only needs basic load and store functionality, so there's
no need to do the extra work to support marking and overlapping stores
correctly yet.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

Add the optional ability to track which entries in an XArray are free
and provide xa_alloc() to replace most of the functionality of the IDR.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

This function reserves a slot in the XArray for users which need
to acquire multiple locks before storing their entry in the tree and
so cannot use a plain xa_store().

Signed-off-by: Matthew Wilcox <willy@infradead.org>

This is documentation on how to use the XArray, not details about its
internal implementation.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Josef Bacik <jbacik@fb.com>