fsck.xfs: fix fsck.xfs run by different shells when fsck.mode=force is set

fsck.xfs: fix fsck.xfs run by different shells when fsck.mode=force is set

When fsck.mode=force is specified in the kernel command line, fsck.xfs
is executed during the boot process. However, when the default shell is
not bash, $PS1 should be a different value, consider the following script:
cat ps1.sh
echo "$PS1"

run ps1.sh with different shells:
ash ./ps1.sh
$
bash ./ps1.sh

dash ./ps1.sh
$
ksh ./ps1.sh

zsh ./ps1.sh

On systems like Ubuntu, where dash is the default shell during the boot
process to improve startup speed. This results in FORCE being incorrectly
set to false and then xfs_repair is not invoked:
if [ -n "$PS1" -o -t 0 ]; then
        FORCE=false
fi

Other distros may encounter this issue too if the default shell is set
to anoother shell.

Check "-t 0" is enough to determine if we are in interactive mode, and
xfs_repair is invoked as expected regardless of the shell used.

Fixes: 04a2d5dc ("fsck.xfs: allow forced repairs using xfs_repair")
Signed-off-by: Gerald Yang <gerald.yang@canonical.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>