Incremental: Use ->validate_geometry instead of ->avail_size

Incremental: Use ->validate_geometry instead of ->avail_size

Since mdadm 3.3 is has not been correct to call ->avail_size if
metadata hasn't been read from the device.  ->validate_geometry
should be used instead.

Unfortunately array_try_spare() didn't get the memo, and it can crash
when adding a spare with no metdata.

So change it to use ->validate_geometry().

Only one place remains that uses ->avail_size(), and that is safe.

Also fix a comment with a typo.

Reported-and-tested-by: Bjørnar Ness <bjornar.ness@gmail.com>
Fixes: 641da7459192 ("super1: separate to version of _avail_space1().")
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>