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: Randy Dunlap <rdunlap@xenotime.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>