kdumpct/fadump: fix ShellCheck findings

This commit fixes ShellCheck findings introduced by the new feature to
pass additional parameters to the fadump kernel. In particular it fixes
the following

In kdumpctl line 971:
                local output=$( { echo "${FADUMP_COMMANDLINE_APPEND}" > "$FADUMP_APPEND_ARGS_SYS_NODE" ; } 2>&1)
                      ^----^ SC2155 (warning): Declare and assign separately to avoid masking return values.

In kdumpctl line 972:
                if [ $? -eq 0 ]; then
                     ^-- SC2181 (style): Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.

Fixes: 53d8e6e ("fadump: fix passing additional parameters for capture kernel")
Fixes: 7b80ce ("fadump: pass additional parameters for capture kernel")
Signed-off-by: Philipp Rudo <[email protected]>

kdumpctl

@@ -970,8 +970,9 @@ check_dump_feasibility()
 fadump_bootargs_append()
 {
 	if [[ -f "$FADUMP_APPEND_ARGS_SYS_NODE" ]]; then
-		local output=$( { echo "${FADUMP_COMMANDLINE_APPEND}" > "$FADUMP_APPEND_ARGS_SYS_NODE" ; } 2>&1)
-		if [ $? -eq 0 ]; then
+		local output
+
+		if output=$( { echo "${FADUMP_COMMANDLINE_APPEND}" > "$FADUMP_APPEND_ARGS_SYS_NODE" ; } 2>&1); then
 			output=$(cat "$FADUMP_APPEND_ARGS_SYS_NODE")
 			dinfo "fadump: additional parameters for capture kernel: '$output'"
 		else