cifs: potential buffer overflow in handling symlinks

Smatch printed a warning:
	arch/x86/crypto/poly1305_glue.c:198 poly1305_update_arch() error:
	__memcpy() 'dctx->buf' too small (16 vs u32max)

It's caused because Smatch marks 'link_len' as untrusted since it comes
from sscanf(). Add a check to ensure that 'link_len' is not larger than
the size of the 'link_str' buffer.

Fixes: c69c1b6e ("cifs: implement CIFSParseMFSymlink()")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/link.c

@@ -85,6 +85,9 @@ parse_mf_symlink(const u8 *buf, unsigned int buf_len, unsigned int *_link_len,
 	if (rc != 1)
 		return -EINVAL;
 
+	if (link_len > CIFS_MF_SYMLINK_LINK_MAXLEN)
+		return -EINVAL;
+
 	rc = symlink_hash(link_len, link_str, md5_hash);
 	if (rc) {
 		cifs_dbg(FYI, "%s: MD5 hash failure: %d\n", __func__, rc);