MD RAID1: Further conditionalize 'fullsync'

A RAID1 device does not necessarily need a fullsync if the bitmap can be used instead.

Similar to commit d6b212f4 in raid5.c, if a raid1
device can be brought back (i.e. from a transient failure) it shouldn't need a
complete resync.  Provided the bitmap is not to old, it will have recorded the areas
of the disk that need recovery.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>