allocate_block fixes.

allocate_block fixes.

Try to set up slightly new rules....

1/ Adding an address as unincorporated requires just a spinlock,
    the inode private_lock
2/ The block with an address being added is not iolock, but is
    Writebehind
3/ incorporation happens under iolock,  removing the list of
    pending addresses also happens here.  so under iolock
    addresses can be added to list but not removed (unless I'm doing
    the removing).

Signed-off-by: NeilBrown <neilb@suse.de>