filemap: fix kernel-doc warnings

Fix filemap.c kernel-doc warnings:

Warning(mm/filemap.c:575): No description found for parameter 'page'
Warning(mm/filemap.c:575): No description found for parameter 'waiter'

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>