sysvshm: pass relevant uap members as arguments

Alter shmget_allocate_segment and shmget_existing to take the values
they want from struct shmget_args rather than passing the struct
around. In general, uap structures should only be the interface to
sys_<foo> functions.

This makes on small functional change and records the allocated space
rather than the requested space. If this turns out to be a problem (e.g.
if software tries to find undersized segments by exact size rather than
using keys), we can correct that easily.

Reviewed by: kib
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27077