mm: purge fragmented percpu vmap blocks

Improve handling of fragmented per-CPU vmaps.  We previously don't free
up per-CPU maps until all its addresses have been used and freed.  So
fragmented blocks could fill up vmalloc space even if they actually had
no active vmap regions within them.

Add some logic to allow all CPUs to have these blocks purged in the case
of failure to allocate a new vm area, and also put some logic to trim
such blocks of a current CPU if we hit them in the allocation path (so
as to avoid a large build up of them).

Christoph reported some vmap allocation failures when using the per CPU
vmap APIs in XFS, which cannot be reproduced after this patch and the
previous bug fix.

Cc: linux-mm@kvack.org
Cc: stable@kernel.org
Tested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Nick Piggin <npiggin@suse.de>
--
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>