genfs_getpages() return unallocated pages using the zero page and
PG_RDONLY; the old code relied on fault logic to get it allocated, which
the direct case can't rely on
instead just allocate the blocks right away; pass PGO_JOURNALLOCKED
so that code wouldn't try to take wapbl lock, this code path is called
with it already held
this should fix KASSERT() due to PG_RDONLY on write with wapbl
towards resolution of PR kern/53124
2386f90fa2