pmemblk.c: fix one logic bug - read always with write

pmemblk.c: fix one logic bug - read always with write

logic issue,if read success and return 0, then pmemblk_write called as well:
if (io_u->ddir == DDIR_READ &&
  0 != pmemblk_read(pmb->pmb_pool, buf, off)) {
	io_u->error = errno;
	break;
} else if (0 != pmemblk_write(pmb->pmb_pool, buf, off)) {
	io_u->error = errno;
	break;
}

Signed-off-by: dennis.wu <dennis.wu@intel.com>