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

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

BugLink: https://bugs.launchpad.net/bugs/1896578
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>
(cherry picked from commit f2e7e269a7525317752d472bb48a549780e87d22)
Signed-off-by: Matthew Ruffell <matthew.ruffell@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>