mm: remove the now-unnecessary mmget_still_valid() hack

mm: remove the now-unnecessary mmget_still_valid() hack

The preceding patches have ensured that core dumping properly takes the
mmap_lock.  Thanks to that, we can now remove mmget_still_valid() and all
its users.

Signed-off-by: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: "Eric W . Biederman" <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Hugh Dickins <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>