ZTS: add mount_loopback to test zfs behind loop dev
Add a test case to reproduce issue #17277:
- Make a pool
- Write a file to the pool
- Mount the file as a loopback device
- Make an XFS filesystem on the loopback device
- Mount the XFS filesystem... <hangs>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Issue #17277
Closes #17329