MFC r357410-r357411: Avoid duplicating VEXEC checks in VOP_CACHEDLOOKUP

r357410: pseudofs: don't do VEXEC check in VOP_CACHEDLOOKUP

VOP_CACHEDLOOKUP should assume that the appropriate VEXEC check has been
done in the caller (vfs_cache_lookup), so it does not belong here.

r357411: 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.