zfs: light refactor to indicate cachedlookup in zfs_lookup

If we come from VOP_CACHEDLOOKUP, we must skip the VEXEC check as it will
have been done in the caller (vfs_cache_lookup). This is a part of D23247,
which may skip the earlier VEXEC check as well if the root fd was opened
with O_SEARCH.

This one required slightly more work as zfs_lookup may also be called
indirectly as VOP_LOOKUP or a couple of other places where we must do the
check.