crypto: essiv - Replace memcpy() + NUL-termination with strscpy()

Use strscpy() to copy the NUL-terminated string 'p' to the destination
buffer instead of using memcpy() followed by a manual NUL-termination.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>