xfs: batch inode reclaim lookup

Batch and optimise the per-ag inode lookup for reclaim to minimise
scanning overhead. This involves gang lookups on the radix trees to
get multiple inodes during each tree walk, and tighter validation of
what inodes can be reclaimed without blocking befor we take any
locks.

This is based on ideas suggested in a proof-of-concept patch
posted by Nick Piggin.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>