erofs: clean up erofs_iget()

isdir indicated REQ_META|REQ_PRIO which no longer works now.
Get rid of isdir entirely.

Link: https://lore.kernel.org/r/20220927063607.54832-2-hsiangkao@linux.alibaba.com


Reviewed-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>