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

[brooks]

Summary

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).

Obtained from: CheriBSD
Effort: CHERI upstreaming
Sponsored by: DARPA

[kib]

May be define some inline function in libc/include/libc_private.h, like

static inline ptrdiff_t
misalign_val(void *p)
{
   return (_ALIGN(p) - p);
}

and use it in all places.

[brooks]

Add an inline function to compute the misalignment. To it in nscache.h
rather than libc_private.h as that's also a private header and I don't
want this sort of thing to spread.