md/raid1: only allocate write behind bio for WriteMostly device

Commit 6607cd31

 ("raid1: ensure write
behind bio has less than BIO_MAX_VECS sectors") tried to guarantee the
size of behind bio is not bigger than BIO_MAX_VECS sectors.

Unfortunately the same calltrace still could happen since an array could
enable write-behind without write mostly device.

To match the manpage of mdadm (which says "write-behind is only attempted
on drives marked as write-mostly"), we need to check WriteMostly flag to
avoid such unexpected behavior.

[1]. https://bugzilla.kernel.org/show_bug.cgi?id=213181#c25

Cc: stable@vger.kernel.org # v5.12+
Cc: Jens Stutte <jens@chianterastutte.eu>
Reported-by: Jens Stutte <jens@chianterastutte.eu>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>