Btrfs: enforce min_bytes parameter during extent allocation

Commit 24542bf7 changed preallocation of
extents to cap the max size we try to allocate.  It's a valid change,
but the extent reservation code is also used by balance, and that
can't tolerate a smaller extent being allocated.

__btrfs_prealloc_file_range already has a min_size parameter, which is
used by relocation to request a specific extent size.  This commit
adds an extra check to enforce that minimum extent size.
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
Reported-by: Stefan Behrens <sbehrens@giantdisaster.de>

fs/btrfs/inode.c

@@ -8502,6 +8502,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 	struct btrfs_key ins;
 	u64 cur_offset = start;
 	u64 i_size;
+	u64 cur_bytes;
 	int ret = 0;
 	bool own_trans = true;
 
@@ -8516,8 +8517,9 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 			}
 		}
 
-		ret = btrfs_reserve_extent(trans, root,
-					   min(num_bytes, 256ULL * 1024 * 1024),
+		cur_bytes = min(num_bytes, 256ULL * 1024 * 1024);
+		cur_bytes = max(cur_bytes, min_size);
+		ret = btrfs_reserve_extent(trans, root, cur_bytes,
 					   min_size, 0, *alloc_hint, &ins, 1);
 		if (ret) {
 			if (own_trans)