tune2fs: kill external journal if device not found

tune2fs: kill external journal if device not found

Continue to remove the external journal device even if the device
cannot be found.

Add a test to verify that the journal device/UUID are actually removed
from the superblock.  It isn't possible to use a real journal device
for testing without loopback devices and such (it must be a block device)
and this would invite complexity and failures in the regression test.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>