ZTS: add mount_loopback to test zfs behind loop dev

Add a test case to reproduce issue #17277:

1. Make a pool
2. Write a file to the pool
3. Mount the file as a loopback device
4. Make an XFS filesystem on the loopback device
5. 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

tests/zfs-tests/include/commands.cfg

@@ -100,6 +100,7 @@ export SYSTEM_FILES_COMMON='awk
     uniq
     vmstat
     wc
+    which
     xargs
     xxh128sum'
 
@@ -146,6 +147,7 @@ export SYSTEM_FILES_LINUX='attr
     lscpu
     lsmod
     lsscsi
+    mkfs.xfs
     mkswap
     modprobe
     mountpoint