ceph: fix page invalidation deadlock

We occasionally want to make a best-effort attempt to invalidate cache
pages without fear of blocking.  If this fails, we fall back to an async
invalidate in another thread.

Use invalidate_mapping_pages instead of invalidate_inode_page2, as that
will skip locked pages, and not deadlock.

Signed-off-by: Sage Weil <sage@newdream.net>