gfs2: fix address space truncation during withdraw

gfs2: fix address space truncation during withdraw

When a withdrawn filesystem's inodes are being evicted, the address spaces of
those inodes still need to be truncated but we can no longer start new
transactions.  We still don't want gfs2_invalidate_folio() to race with
gfs2_log_flush(), so take a read lock on sdp->sd_log_flush_lock in that case.
(It may not be obvious, but gfs2_invalidate_folio() is a jdata-only address
space operation.)

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