fstests: use udevadm settle after pvremove

fstests: use udevadm settle after pvremove

As with creation, we also need to use udevadm settle
when removing a pv, otherwise we can trip on races with
module removals for the block devices in use.

This reduces the amount of time in which a block device
module refcnt for test modules such as scsi_debug spends
outside of 0.

Races with the refcnt being greater than 0 means module
removal can fail causing false positives. This helps
ensure that the pv is really long gone. These issues
are tracked for scsi_debug [0] and later found to be a
generic issue regardless of filesystem with pvremove [1].

Using udevadm settle *helps*, it does not address all
possible races with the refcnt as noted in the generic
bug entry [1].

[0] https://bugzilla.kernel.org/show_bug.cgi?id=212337
[1] https://bugzilla.kernel.org/show_bug.cgi?id=214015

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>