md/raid10: pull the code that wait for blocked dev into one function

The following patch will reuse these logics, so pull the same codes into
one function.

Tested-by: Adrian Huang <ahuang12@lenovo.com>
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>