vm: fix swap reservation leak and clean up surrounding code

vm: fix swap reservation leak and clean up surrounding code

The code did not subtract from the global counter if per-uid reservation
failed.

Cleanup highlights:
- load overcommit once
- move per-uid manipulation to dedicated routines
- don't fetch wire count if requested size is below the limit
- convert return type from int to bool
- ifdef the routines with _KERNEL to keep vm.h compilable by userspace

Reviewed by:	kib (previous version)
Differential Revision:	https://reviews.freebsd.org/D25787