kexec: i386: bzImage: fix memory leak caused by get_command_line

Since get_command_line returns dynamically allocated memory, it is
easy for the caller to forget freeing the memory. Here fixes a
memory leak caused by this function.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: Simon Horman <horms@verge.net.au>