btrfs: set AS_KERNEL_FILE on the btree_inode

btrfs: set AS_KERNEL_FILE on the btree_inode

extent_buffers are global and shared so their pages should not belong to
any particular cgroup (currently whichever cgroups happens to allocate the
extent_buffer).

Btrfs tree operations should not arbitrarily block on cgroup reclaim or
have the shared extent_buffer pages on a cgroup's reclaim lists.

Link: https://lkml.kernel.org/r/2ee99832619a3fdfe80bf4dc9760278662d2d746.1755812945.git.boris@bur.io
Signed-off-by: Boris Burkov <boris@bur.io>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Tested-by: syzbot@syzkaller.appspotmail.com
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qu Wenruo <wqu@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>