doesn't have the exec bit set, we need to have PROT_EXEC set
in order for some expected mmap/mprotect behavior to work, so
do the last bit slightly differently: if udv_attach() fails, and
the protection (NOT maxprot) doens't include PROT_EXEC, then clear
PROT_EXEC from maxprot and try udv_attach() again.
Sigh, mmap really needs to be rototilled.