tmpfs: push VEXEC check into tmpfs_lookup()

vfs_cache_lookup() has already done the appropriate VEXEC check, therefore
we must not re-check in VOP_CACHEDLOOKUP.

This fixes O_SEARCH semantics on tmpfs and removes a redundant descent into
VOP_ACCESS() in the common case.

Reported-by: arichardson (via CheriBSD Jenkins CI)
Reviewed-by: kib
MFC-after: 3 days
Differential Revision: https://reviews.freebsd.org/D28401