get*ent: be consistant about _ALIGN(p) - p

Add an nscache specific inline function to calculate the misalignment
rather than adding and subtracting _ALIGN(p) and p which can take the
buffer far out of bound (undefined behavior in C and unsupported on
CHERI).

Reviewed by: kib
Effort: CHERI upstreaming
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D53945