ext4: Convert ext4 to new truncate calling convention

Trivial conversion.  Fixup one error handling case calling vmtruncate()
and remove ->truncate callback. We also fix a bug that IS_IMMUTABLE and
IS_APPEND files could not be truncated during failed writes. In fact, the
test can be completely removed as upper layers do necessary permission
checks for truncate in do_sys_[f]truncate() and may_open() anyway.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>