Bug#787950: mdadm: checkarray doesn't work with dash

[Uoti Urpala]

This is a kernel/dash issue. In kernel 4.0, the md sysfs implementation
does not work with partial reads from /sys/block/md0/md/sync_action;
the read syscall returns the entire contents of the file even if you
try to read less. Dash has an inefficient implementation which reads
input one byte at a time, and fails when it hits the kernel
misbehavior.