xfrm: xfrm_user: use strscpy() for alg_name

Replace the strcpy() calls that copy the canonical algorithm name into
alg_name with strscpy() to avoid potential overflows and guarantee NULL
termination.

Destination is alg_name in xfrm_algo/xfrm_algo_auth/xfrm_algo_aead
(size CRYPTO_MAX_ALG_NAME).

Tested in QEMU (BusyBox/Alpine rootfs):
 - Added ESP AEAD (rfc4106(gcm(aes))) and classic ESP (sha256 + cbc(aes))
 - Verified canonical names via ip -d xfrm state
 - Checked IPComp negative (unknown algo) and deflate path

Signed-off-by: Miguel García <miguelgarciaroman8@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>