btrfs: update comment for struct btrfs_inode::lock

Update the comment for the lock named "lock" in struct btrfs_inode because
it does not mention that the fields "delalloc_bytes", "defrag_bytes",
"csum_bytes", "outstanding_extents" and "disk_i_size" are also protected
by that lock.

Also add a comment on top of each field protected by this lock to mention
that the lock protects them.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>