mm/gup.c: fix follow_page_mask() kerneldoc comment

mm/gup.c: fix follow_page_mask() kerneldoc comment

Commit df06b37ffe5a ("mm/gup: cache dev_pagemap while pinning pages")
modified the signature of follow_page_mask() but left the parameter
description behind.

Update the description to make the code and comments agree again.

While at it, update formatting of the return value description to match
Documentation/doc-guide/kernel-doc.rst guidelines.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Rapoport <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>