Limit the number of entries allocated for a REPORT_ZONES command.

The DIOCGETZONE ioctl can be used to fetch the zone list of an SMR
drive, and the caller specifies the number of entries it wants to fetch.
Clamp the caller's request to a sane limit so that a user cannot attempt
large allocations. Callers already need to invoke the ioctl multiple
times to fetch the full list in general, so there's no harm in limiting
the number of entries returned.

Fix style while here.

admbug: 807
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: asomers, ken
Tested by: ken
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19249