gfs2: don't stop reads while withdraw in progress

When gfs2 withdraws a file system, it calls signal_our_withdraw which
triggers another node to replay the withdrawing node's journal. Then it
waits until it knows the journal has been replayed. Part of this wait is
to repeatedly call check_journal_clean which calls gfs2_jdesc_check,
which checks to see if the journal is sane. As part of its sanity checks
it needs to re-read its journal's metadata. But with today's code, any
attempt to re-read the metadata results in -EIO because of a check for
the file system withdraw in function gfs2_meta_wait.

This patch adds an additional check for SDF_WITHDRAW_IN_PROG, to tell
if the read is done while the withdraw is in progress. In that case
we allow the metadata read to not be rejected. Therefore the metadata
check is done properly, so the withdraw sequence can finish normally.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>