fs/dcache.c: fix new kernel-doc warning

Fix new kernel-doc warning in fs/dcache.c:

  Warning(fs/dcache.c:797): No description found for parameter 'sb'
Signed-off-by: NRandy Dunlap <rdunlap@xenotime.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -795,6 +795,7 @@ static void __shrink_dcache_sb(struct super_block *sb, int count, int flags)
 
 /**
  * prune_dcache_sb - shrink the dcache
+ * @sb: superblock
  * @nr_to_scan: number of entries to try to free
  *
  * Attempt to shrink the superblock dcache LRU by @nr_to_scan entries. This is