sysvshm: pass relevant uap members as arguments

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