Btrfs: remove lock assert from get_restripe_target()

This fixes a regression introduced by fc67c45.  spin_is_locked() always
returns 0 on UP kernels, which caused assert in get_restripe_target() to
be fired on every call from btrfs_reduce_alloc_profile() on UP systems.
Remove it completely for now, it's not clear if it's going to be needed
in future.

Reported-by: Bobby Powers <bobbypowers@gmail.com>
Reported-by: Mitch Harder <mitch.harder@sabayonlinux.org>
Tested-by: Mitch Harder <mitch.harder@sabayonlinux.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -3152,15 +3152,14 @@ static void set_avail_alloc_bits(struct btrfs_fs_info *fs_info, u64 flags)
 /*
  * returns target flags in extended format or 0 if restripe for this
  * chunk_type is not in progress
+ *
+ * should be called with either volume_mutex or balance_lock held
  */
 static u64 get_restripe_target(struct btrfs_fs_info *fs_info, u64 flags)
 {
 	struct btrfs_balance_control *bctl = fs_info->balance_ctl;
 	u64 target = 0;
 
-	BUG_ON(!mutex_is_locked(&fs_info->volume_mutex) &&
-	       !spin_is_locked(&fs_info->balance_lock));
-
 	if (!bctl)
 		return 0;