seltests/zram: fix syntax error

Not all shells define a variable UID. This is a bash and zsh feature only.
In other shells, the UID variable is not defined, so here test command
expands to [ != 0 ] which is a syntax error.

Without this patch:
root@HGH1000007090:/opt/work/linux/tools/testing/selftests/zram# sh zram.sh
zram.sh: 8: [: !=: unexpected operator
zram.sh : No zram.ko module or /dev/zram0 device file not found
zram.sh : CONFIG_ZRAM is not set

With this patch:
root@HGH1000007090:/opt/work/linux/tools/testing/selftests/zram# sh ./zram.sh
zram.sh : No zram.ko module or /dev/zram0 device file not found
zram.sh : CONFIG_ZRAM is not set

Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>