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

md/raid10: pull codes 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.

Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
(cherry picked from commit f046f5d0d79cdb968f219ce249e497fd1accf484)
Signed-off-by: Matthew Ruffell <matthew.ruffell@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>
Signed-off-by: Ian May <ian.may@canonical.com>