libxfs: unmap xmbuf pages to avoid disaster

libxfs: unmap xmbuf pages to avoid disaster

It turns out that there's a maximum mappings count, so we need to be
smartish about not overflowing that with too many xmbuf buffers.  This
needs to be a global value because high-agcount filesystems will create
a large number of xmbuf caches but this is a process-global limit.

Cc: <linux-xfs@vger.kernel.org> # v6.9.0
Fixes: 124b388dac17f5 ("libxfs: support in-memory buffer cache targets")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>