pam_zfs_key: malloc and mlock/munlock won't match

mlock(2) and munlock(2) operate on memory pages whereas malloc(3)
does not. So if you munlock(2) a malloced memory region, the whole
page containing it is freed. Since this page may contain another
malloced and mlocked memory region, used as a password buffer by a
concurrent running instance of pam_zfs_key, there is a slight chance
of leaking passwords. By using mmap(2) we avoid such problems since
it will return whole pages on page aligned addresses.

Although the above concern may be mostly academical, it is still
better to use mmap(2) for allocating memory since the FreeBSD
documentation suggests to call mlock(2) and munlock(2) on page
aligned addresses, and other implementations even require it.

While here, remove duplicate code in alloc_pw_string() by calling
alloc_pw_size().

Reviewed-by: Felix Dörre <felix@dogcraft.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #12665