DIOCGSECTORSIZE expects to write to a u_int, but struct zfs_probe_args
member secsz was a uint16_t

sys/boot/zfs/zfs.c has a probe args structure member, secsz, that is a
uint16_t for media sector size; it is used as an argument for ioctl()
at line 484. however, this ioctl writes 32 bits of data (u_int *) and
therefore this ioctl will overwrite and corrupt 16 bits of memory.
other use cases seem to use correct u_int type for secsz.

PR: 204358
Submitted by: Toomas Soome <tsoome at me.com>
Reviewed by: asomers, delphij, smh
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D4811