namei: preserve errors from fget_cap_locked

Most notably, we want to make sure we don't clobber any capabilities-related
errors. This is a regression from r357412 (O_SEARCH) that was picked up by
the capsicum tests.

PR: 243839
Reviewed by: kib (committed form recommended by)
Tested by: lwhsu
Differential Revision: https://reviews.freebsd.org/D23479