raid10: refactor common wait code from regular read/write request

Both raid10_read_request and raid10_write_request share
the same code at the beginning of them, so introduce
regular_request_wait to clean up code, and call it in
both request functions.

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>