btrfs: polish names of kmem caches

Usecase:

  watch 'grep btrfs < /proc/slabinfo'

easy to watch all caches in one go.

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --cc fs/btrfs/delayed-inode.c
Simple merge

diff --cc fs/btrfs/extent_io.c
Simple merge

diff --cc fs/btrfs/extent_map.c
Simple merge

diff --cc fs/btrfs/inode.c
Simple merge