btrfs: use may_create_dentry() in btrfs_mksubvol()

There is no longer the need to use btrfs_may_create(), which was a copy
of the VFS private function may_create(), since now that functionality
is exported by the VFS as a function named may_create_dentry(). So change
btrfs_mksubvol() to use the VFS function and remove btrfs_may_create().

Note that the btrfs copy was missing an audit_inode_child() call that we
have in the VFS function. This only reinforces the need to use a common
function, as it's very easy for the btrfs copy to get out of sync and
therefore a maintenance burden.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Link: https://patch.msgid.link/adf8c802c77ec1c855ea9fe12491120ccc29a294.1768307858.git.fdmanana@suse.com
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: fdmanana

fs/btrfs/ioctl.c

@@ -815,19 +815,6 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 	return ret;
 }
 
-/* copy of may_create in fs/namei.c() */
-static inline int btrfs_may_create(struct mnt_idmap *idmap,
-				   struct inode *dir, const struct dentry *child)
-{
-	if (d_really_is_positive(child))
-		return -EEXIST;
-	if (IS_DEADDIR(dir))
-		return -ENOENT;
-	if (!fsuidgid_has_mapping(dir->i_sb, idmap))
-		return -EOVERFLOW;
-	return inode_permission(idmap, dir, MAY_WRITE | MAY_EXEC);
-}
-
 /*
  * Create a new subvolume below @parent.  This is largely modeled after
  * sys_mkdirat and vfs_mkdir, but we only do a single component lookup
@@ -849,7 +836,7 @@ static noinline int btrfs_mksubvol(struct dentry *parent,
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
 
-	ret = btrfs_may_create(idmap, dir, dentry);
+	ret = may_create_dentry(idmap, dir, dentry);
 	if (ret)
 		goto out_dput;