Eliminate unnecessary loop in _cap_check()

Calling cap_rights_contains() several times with the same inputs is not
going to produce a different output. The variable being iterated, i, is
never used inside the for loop.

The loop is actually done in cap_rights_contains()

Submitted by: Ryan Moeller <ryan@freqlabs.com>
Reviewed by: oshogbo, ed
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D7369