[PATCH] blockgroup_lock: hashed spinlocks for ext2 and ext3

[PATCH] blockgroup_lock: hashed spinlocks for ext2 and ext3

ext2 and ext3 per-blockgroup metadata needs locking.  An fs-wide lock is
expensive, and a per-blockgroup lock consumes too much storage (up to 32768
blockgroups per filesystem).  We need something in-between.

blockgroup_locks are very simple hashed spinlocks which provide this
compromise.  The size of the lock is scaled by NR_CPUS to implement an
additional speed/space tradeoff.

These locks are actually fairly generic.  However I presented it as something
which is specific to ext2 and ext3 so that people wouldn't go using them all
over the place.  They consume a lot of storage.