loader: F_READ/F_WRITE should be checked against masked flag

The work to make it possible to avoid bcache via using F_NORA modifier did
miss the fact that not all loader platforms are using the bcache, and so
it is possible the modifier is not cleared, as bcache strategy function is
not used.

For fix, we make sure the checks are dont with masked flag.

This patch does fix boot for platforms which do not use bcache.

Reported by: emaste
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D10422