staging: erofs: tidy up zpvec.h

- use shorter function names:
  z_erofs_pagevec_enqueue and z_erofs_pagevec_dequeue;
- minor code cleanup.

In order to keep in line with erofs-outofstaging patchset.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-16-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>