Fix loader bug causing too many pages allocation when bootloader is U-Boot

FreeBSD loader expects to have mmsz variable set by bootloader.
U-Boot behaviour is that if buffer size is not big enough to keep
whole memory map, assign the smallest correct buffer size to sz
and return error.

In other words U-Boot assumes that nobody will need mmsz value when buffer
is not filled with memory map, which is not true, so calculated pages value
was too big to allocate.

Solution: Simply assign default value to mmsz.

Submitted by: Patryk Duda <pdk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12194