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>

tools/testing/selftests/zram/zram.sh

 #!/bin/bash
 TCID="zram.sh"
 
-check_prereqs()
-{
-	local msg="skip all tests:"
-
-	if [ $UID != 0 ]; then
-		echo $msg must be run as root >&2
-		exit 0
-	fi
-}
+. ./zram_lib.sh
 
 run_zram () {
 echo "--------------------"

tools/testing/selftests/zram/zram_lib.sh

@@ -23,8 +23,9 @@ trap INT
 check_prereqs()
 {
 	local msg="skip all tests:"
+	local uid=$(id -u)
 
-	if [ $UID != 0 ]; then
+	if [ $uid -ne 0 ]; then
 		echo $msg must be run as root >&2
 		exit 0
 	fi