selftests/net: fix EVP_MD_CTX leak in tcp_mmap

selftests/net: fix EVP_MD_CTX leak in tcp_mmap

In tcp_mmap.c, both child_thread() and main() allocate an EVP_MD_CTX
via EVP_MD_CTX_new() when integrity checking is enabled, but neither
function releases the context.  child_thread() misses the free in its
common cleanup block, and main() returns without freeing the context.

This results in a SHA256 context leak on every run that uses the
‑i (integrity) option.  Add the missing EVP_MD_CTX_free() calls to
the appropriate cleanup paths to fix the leak.

Fixes: 5c5945dc695c ("selftests/net: Add SHA256 computation over data sent in tcp_mmap")
Signed-off-by: Wang Yan <wangyan01@kylinos.cn>
Link: https://patch.msgid.link/20260702025949.442523-1-wangyan01@kylinos.cn
Signed-off-by: Paolo Abeni <pabeni@redhat.com>