xfs: avoid memory allocation under m_peraglock in growfs code

Allocate the memory for the larger m_perag array before taking the
per-AG lock as the per-AG lock can be taken under the i_lock which
can be taken from reclaim context.

Reported by the new reclaim context tracing in lockdep.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Felix Blyakher <felixb@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>