gfs2: Disable page faults during lockless buffered reads

During lockless buffered reads, filemap_read() holds page cache page
references while trying to copy data to the user-space buffer.  The
calling process isn't holding the inode glock, but the page references
it holds prevent those pages from being removed from the page cache, and
that prevents the underlying inode glock from being moved to another
node.  Thus, we can end up in the same kinds of distributed deadlock
situations as with normal (non-lockless) buffered reads.

Fix that by disabling page faults during lockless reads as well.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>