misc/fuse2fs: avoid error-prone strncpy() pattern

misc/fuse2fs: avoid error-prone strncpy() pattern

'strncpy(dst, src, strlen(src))' is usually wrong, as it doesn't copy
the null terminator.  For this reason, it causes a -Wstringop-truncation
warning with gcc 8 and later.

The code happens to be correct anyway, since the destination buffer is
zero-initialized.  But to avoid relying on this, let's just copy the
terminating null.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>