gfs2: Clean up the error handling in gfs2_page_mkwrite

We're setting an error number so that block_page_mkwrite_return
translates it into the corresponding VM_FAULT_* code in several places,
but this is getting confusing, so set the VM_FAULT_* codes directly
instead.  (No change in functionality.)

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>