idmapped-mounts: switch to getopt_long_only()

idmapped-mounts: switch to getopt_long_only()

We're not using the shortopts anywhere anyway  and shouldn't encourage
using shortopts.  It's much more descriptive to see:

$here/src/idmapped-mounts/idmapped-mounts \
        --test-btrfs \
        --device "$TEST_DEV" \
        --mountpoint "$TEST_DIR" \
        --scratch-device "$SCRATCH_DEV" \
        --scratch-mountpoint "$SCRATCH_MNT"
        --fstype "$FSTYP"

in a test than it is to see:

$here/src/idmapped-mounts/idmapped-mounts \
        -b
        -d "$TEST_DEV" \
        -m "$TEST_DIR" \
        -s "$SCRATCH_DEV" \
        -a "$SCRATCH_MNT" \
        -f "$FSTYP"

In the second case one has to go consult the source code to make sure
that the correct option is passed. In the first case one can just see it
directly.

Cc: fstests@vger.kernel.org
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>