mm/mlock: drop dead code in count_mm_mlocked_page_nr()

The check for mm being null has never been needed since the only caller
has always passed in current->mm.  Remove the check from
count_mm_mlocked_page_nr().

Link: https://lkml.kernel.org/r/20220615174050.738523-1-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>