fix filemap.c kernel-doc

Fix kernel-doc warning:
Warning(linux-2.6.23-rc1-mm1//mm/filemap.c:864): No description found for parameter 'ra'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>