gfs2: Fix withdraw race

Function gfs2_withdraw() tries to synchronize concurrent callers by
atomically setting the SDF_WITHDRAWN flag in the first caller, setting
the SDF_WITHDRAW_IN_PROG flag to indicate that a withdraw is in
progress, performing the actual withdraw, and clearing the
SDF_WITHDRAW_IN_PROG flag when done.  All other callers wait for the
SDF_WITHDRAW_IN_PROG flag to be cleared before returning.

This leaves a small window in which callers can find the SDF_WITHDRAWN
flag set before the SDF_WITHDRAW_IN_PROG flag has been set, causing them
to return prematurely, before the withdraw has been completed.

Fix that by setting the SDF_WITHDRAWN and SDF_WITHDRAW_IN_PROG flags
atomically.

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