linux-user: do_setsockopt: fix SOL_ALG.ALG_SET_KEY

This setsockopt accepts zero-lengh optlen (current qemu implementation
does not allow this).  Also, there's no need to make a copy of the key,
it is enough to use lock_user() (which accepts zero length already).

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2197
Fixes: f31dddd "linux-user: Add support for setsockopt() option SOL_ALG"
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Message-Id: <20240331100737.2724186-2-mjt@tls.msk.ru>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 04f6fb8)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/syscall.c

@@ -2277,18 +2277,13 @@ static abi_long do_setsockopt(int sockfd, int level, int optname,
         switch (optname) {
         case ALG_SET_KEY:
         {
-            char *alg_key = g_malloc(optlen);
-
+            char *alg_key = lock_user(VERIFY_READ, optval_addr, optlen, 1);
             if (!alg_key) {
-                return -TARGET_ENOMEM;
-            }
-            if (copy_from_user(alg_key, optval_addr, optlen)) {
-                g_free(alg_key);
                 return -TARGET_EFAULT;
             }
             ret = get_errno(setsockopt(sockfd, level, optname,
                                        alg_key, optlen));
-            g_free(alg_key);
+            unlock_user(alg_key, optval_addr, optlen);
             break;
         }
         case ALG_SET_AEAD_AUTHSIZE: