mm/mlock: rename mlock_future_check() to mlock_future_ok()

It is felt that the name mlock_future_check() is vague - it doesn't
particularly convey the function's operation.  mlock_future_ok() is a
clearer name for a predicate function.

Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>