btrfs: add lockdep_assert_held to need_preemptive_reclaim

In a previous patch ("btrfs: extend locking to all space_info members
accesses") the locking for the space_info members was extended in
btrfs_preempt_reclaim_metadata_space because not all the member
accesses that needed locks were actually locked (bytes_pinned et al).

It was then suggested to also add a call to lockdep_assert_held to
need_preemptive_reclaim. This function also works with space_info
members. As of now, it has only two call sites which both hold the lock.
Suggested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -737,6 +737,8 @@ static bool need_preemptive_reclaim(struct btrfs_fs_info *fs_info,
 	u64 thresh = div_factor_fine(space_info->total_bytes, 90);
 	u64 used;
 
+	lockdep_assert_held(&space_info->lock);
+
 	/* If we're just plain full then async reclaim just slows us down. */
 	if ((space_info->bytes_used + space_info->bytes_reserved +
 	     global_rsv_size) >= thresh)