fuse: Forward vfs_fadvise to backing_file in passthrough mode

When an application invokes posix_fadvise(..., POSIX_FADV_DONTNEED) or
other fadvise advice on a FUSE passthrough file descriptor, the VFS
currently only executes generic_fadvise() on the upper FUSE inode
mapping.

Because FUSE passthrough does not forward fadvise operations to the
underlying backing file, pages residing in the lower filesystem's
pagecache remain resident in memory.

During media-heavy workloads (such as image thumbnail generation or
streaming), single-access file data accumulates in the inactive pagecache
of the lower filesystem. Under memory pressure, the kernel retains these
stale media pages and instead evicts active executable pages from
running processes, leading to severe direct reclaim latency spikes.

Architecturally, Linux VFS supports filesystem-specific fadvise
delegation via the .fadvise hook in struct file_operations. Implement
fuse_file_fadvise() in fs/fuse/file.c and register it in
fuse_file_operations. When passthrough is active on a fuse_file,
fuse_file_fadvise() forwards the vfs_fadvise() call to the lower
backing_file under the opened credentials, allowing POSIX_FADV_DONTNEED
to cleanly invalidate the physical backing filesystem pagecache.

Signed-off-by: Shai Barack <shayba@google.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Bernd Schubert <bschubert@ddn.com>
Cc: Sandeep Dhavale <dhavale@google.com>
Cc: Akilesh Kailash <akailash@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: fuse-devel@lists.linux.dev
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

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