ext4: fix -Wstringop-truncation warnings

The strncpy() function may create a unterminated string,
use strscpy_pad() instead.

This fixes the following warning:

fs/ext4/super.c: In function '__save_error_info':
fs/ext4/super.c:368:2: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
  strncpy(es->s_last_error_func, func, sizeof(es->s_last_error_func));
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fs/ext4/super.c:373:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
   strncpy(es->s_first_error_func, func,
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    sizeof(es->s_first_error_func));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Wenlin Kang <wenlin.kang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>