udf: Clarify meaning of f_files in udf_statfs

UDF does not have separate preallocated table of inodes. So similarly to
XFS we pretend that every free block is also a free inode in statfs(2)
output. Clarify this in a comment.
Signed-off-by: NJan Kara <jack@suse.cz>

fs/udf/super.c

@@ -2402,6 +2402,10 @@ static int udf_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_blocks = sbi->s_partmaps[sbi->s_partition].s_partition_len;
 	buf->f_bfree = udf_count_free(sb);
 	buf->f_bavail = buf->f_bfree;
+	/*
+	 * Let's pretend each free block is also a free 'inode' since UDF does
+	 * not have separate preallocated table of inodes.
+	 */
 	buf->f_files = (lvidiu != NULL ? (le32_to_cpu(lvidiu->numFiles) +
 					  le32_to_cpu(lvidiu->numDirs)) : 0)
 			+ buf->f_bfree;