fsx: fix strncpy usage error

fsx: fix strncpy usage error

We shouldn't feed sizeof() to strncpy as the string length.  Just use
snprintf, which at least doesn't have the zero termination problems.

In file included from /usr/include/string.h:495,
                 from ../src/global.h:73,
                 from fsx.c:16:
In function 'strncpy',
    inlined from 'main' at fsx.c:2944:5:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: warning:
'__builtin_strncpy' specified bound 4096 equals destination size
[-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function 'strncpy',
    inlined from 'main' at fsx.c:2914:4:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: warning:
'__builtin_strncpy' specified bound 1024 equals destination size
[-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>