eal: fix leak in shared lib mode detection

eal: fix leak in shared lib mode detection

This is reported by our internal covscan:

1. dpdk-20.11/lib/librte_eal/common/eal_common_options.c:508: alloc_fn:
Storage is returned from allocation function "dlopen".
6. dpdk-20.11/lib/librte_eal/common/eal_common_options.c:508:
leaked_storage: Failing to save or free storage allocated by
"dlopen("librte_eal.so.21.0", 5)" leaks it.

 #   506|   	 * shared library is not already loaded i.e. it's
 #   statically linked.)
 #   507|   	 */
 #   508|-> 	if (dlopen("librte_eal.so."ABI_VERSION, RTLD_LAZY |
 #   RTLD_NOLOAD) != NULL &&
 #   509|   			*default_solib_dir != '\0' &&
 #   510|   			stat(default_solib_dir, &sb) == 0 &&

This leak is not an issue per se, but on the other hand, this is easy
to fix and I prefer not having to waive this warning later.

Fixes: 06c7871dde01 ("eal: restrict default plugin path to shared lib mode")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>