fs/iso9660: Fix invalid free

fs/iso9660: Fix invalid free

The ctx->filename can point to either a string literal or a dynamically
allocated string. The ctx->filename_alloc field is used to indicate the
type of allocation.

An issue has been identified where ctx->filename is reassigned to
a string literal in susp_iterate_dir() but ctx->filename_alloc is not
correctly handled. This oversight causes a memory leak and an invalid
free operation later.

The fix involves checking ctx->filename_alloc, freeing the allocated
string if necessary and clearing ctx->filename_alloc for string literals.

Reported-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>