misc/e4defrag: fix -Wstringop-truncation warnings

misc/e4defrag: fix -Wstringop-truncation warnings

Fix two -Wstringop-truncation warnings in is_ext4() by simplifying how
how mnt_type is handled and by using the correct bound for mnt_fsname.

Fix a -Wstringop-truncation warning in main() by replacing the fragile
pattern 'strncpy(dst, src, strnlen(src, N))', which doesn't
null-terminate the destination string, with a standard string copy.  (It
happened to work anyway because dst happens to be zero-initialized.)

These warnings showed up when building with -Wall with gcc 8 or later.

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