af_alg: recover better from crypto failures * lib/af_alg.c (afalg_stream): Recover from crypto failures if the input stream is seekable, by repositioning the stream back to where it was, possibly by just calling sendfile with an offset arg. This lets us return -EAFNOSUPPORT instead of -EIO in some cases, which lets our callers try again with user-mode code. * modules/crypto/af_alg (Depends-on): Depend on fseeko and ftello instead of on fflush and lseek.