Non on-pagefault code should not access any not-mmapped memory.

Here we just refactor the code we already had to block/restore
SIGSEGV from fileh_dirty_writeout() and use it in all functions called
from non-pagefaulting context, as promised.

This way, if there is an error in virtmem implementation which
incorrectly accesses prepared for BigFile maps memory, we'll just die
with coredump instead of trying to incorrectly handle the pagefault.