ksmbd: Replace strcpy + strcat to improve convert_to_nt_pathname

ksmbd: Replace strcpy + strcat to improve convert_to_nt_pathname

strcpy() is deprecated [1] and using strcat() is discouraged. Replace
them by assigning the prefix directly and by using memcpy() to copy the
pathname. Using memcpy() is safe because we already know the length of
the source string and that it is guaranteed to be NUL-terminated.

Allocate only as many bytes as needed and replace kzalloc() with
kmalloc() since memcpy() overwrites the entire buffer anyway.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strcpy
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>