xfs: use btree block initialisation functions in growfs

Factor xfs_btree_init_block() to be independent of the btree cursor,
and use the function to initialise btree blocks in the growfs code.
This makes adding support for different format btree blocks simple.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>