raid10: increment write counter after bio is split

md pending write counter must be incremented after bio is split,
otherwise it gets decremented too many times in end bio callback and
becomes negative.

Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Reviewed-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>