vfs_cache.c: Use CACHE_FPL_SUPPORTED_CN_FLAGS

Commit 2ec2ba7e232d added some code to cache_can_fplookup()
which worked (ensuring an abort when OPENNNAMED was set),
but showed I didn't understand what
CACHE_FPL_SUPPORTED_CN_FLAGS was used for.

This patch cleans it up.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50524
Fixes: 2ec2ba7e232d ("vfs: Add VFS/syscall support for Solaris style extended attributes")