The background inode inactivation can attached dquots to inodes, but
this can race with a foreground quotacheck failure that leads to
disabling quotas and freeing the mp->m_quotainfo structure. The
background inode inactivation then tries to allocate a quota, tries
to dereference mp->m_quotainfo, and crashes like so:

XFS (loop1): Quotacheck: Unsuccessful (Error -5): Disabling quotas.
xfs filesystem being mounted at /root/syzkaller.qCVHXV/0/file0 supports timestamps until 2038 (0x7fffffff)
BUG: kernel NULL pointer dereference, address: 00000000000002a8
....
CPU: 0 PID: 161 Comm: kworker/0:4 Not tainted 6.2.0-c9c3395d #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
Workqueue: xfs-inodegc/loop1 xfs_inodegc_worker
RIP: 0010:xfs_dquot_alloc+0x95/0x1e0
....
Call Trace:
 <TASK>
 xfs_qm_dqread+0x46/0x440
 xfs_qm_dqget_inode+0x154/0x500
 xfs_qm_dqattach_one+0x142/0x3c0
 xfs_qm_dqattach_locked+0x14a/0x170
 xfs_qm_dqattach+0x52/0x80
 xfs_inactive+0x186/0x340
 xfs_inodegc_worker+0xd3/0x430
 process_one_work+0x3b1/0x960
 worker_thread+0x52/0x660
 kthread+0x161/0x1a0
 ret_from_fork+0x29/0x50
 </TASK>
....

Prevent this race by flushing all the queued background inode
inactivations pending before purging all the cached dquots when
quotacheck fails.
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>