Btrfs: Fix an off by one in the extent_map prepare write code

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index a9f5d6d..5f5b7b8 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1956,7 +1956,7 @@
 
 	mutex_lock(&root->fs_info->fs_mutex);
 	err = btrfs_check_free_space(root, PAGE_CACHE_SIZE, 0);
-	mutex_lock(&root->fs_info->fs_mutex);
+	mutex_unlock(&root->fs_info->fs_mutex);
 	if (err)
 		return -ENOSPC;