mm/memfd: replace deprecated strcpy() with memcpy() in alloc_name()

mm/memfd: replace deprecated strcpy() with memcpy() in alloc_name()

strcpy() is deprecated; use memcpy() instead.

Not copying the NUL terminator is safe because strncpy_from_user() would
overwrite it anyway by appending uname to the destination buffer at index
MFD_NAME_PREFIX_LEN.

No functional changes intended.

Link: https://github.com/KSPP/linux/issues/88
Link: https://lkml.kernel.org/r/20250712174516.64243-2-thorsten.blum@linux.dev
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>