kdumpctl: fix infinite loop caused by running under bash

Description of problem
(https://bugzilla.redhat.com/show_bug.cgi?id=1465735):
Run `kdumpctl status` as normal user, get below error messages:

Another app is currently holding the kdump lock; waiting for it to exit...
flock: 9: Bad file descriptor
Another app is currently holding the kdump lock; waiting for it to exit...
flock: 9: Bad file descriptor
...

The bug is caused by behavior difference between bash
and sh (bash in posix).

In the function single_instance_lock in kdumpctl script,
there is

exec 9>/var/lock/kdump

which will fail in user mode. However, this fail will cause
script exiting under bash but not exiting under sh, causing
infinite loop because the flock will always fail.

According to the 16th item in
ftp://ftp.gnu.org/old-gnu/Manuals/bash-2.02/html_node/bashref_66.html

If a POSIX.2 special builtin returns an error status, a non-
interactive shell exits.

And according to
https://www.gnu.org/software/bash/manual/html_node/Special-Builtins.html

exec is one of the POSIX.2 special builtin's.

This patch fixes the bug by checking exec return value.

Fixes: 9fb2996d05c2 ("kdumpctl: change the shebang header to use /bin/bash")
Signed-off-by: Ziyue Yang <ziyang@redhat.com>
Acked-by: Dave Young <dyoung@redhat.com>
Reviewed-by: Xunlei Pang <xlpang@redhat.com>

kdumpctl

@@ -37,6 +37,10 @@ single_instance_lock()
 	local rc timeout=5
 
 	exec 9>/var/lock/kdump
+	if [ $? -ne 0 ]; then
+		echo "Create file lock failed"
+		exit 1
+	fi
 
 	flock -n 9
 	rc=$?