Mostly trivial conversion. We fix a bug that IS_IMMUTABLE and IS_APPEND files
could not be truncated during failed writes as we change the code.  In fact the
test is not needed at all because both IS_IMMUTABLE and IS_APPEND is tested in
upper layers in do_sys_[f]truncate(), may_write(), etc.
Signed-off-by: Jan Kara <jack@suse.cz>