lightnvm: pblk: stop recreating global caches

Pblk should not create a set of global caches every time
a pblk instance is created. The global caches should be
made available only when there is one or more pblk instances.

This patch bundles the global caches together with a kref
keeping track of whether the caches should be available or not.

Also, turn the global pblk lock into a mutex that explicitly
protects the caches (as this was the only purpose of the lock).

Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>