xlog: fix xdir_say_gc() usage in xdir_collect_garbage()

xdir_say_gc() takes errno and return code of unlink() sys call. If RC
is negative (meaning that unlink failed) we reset errno to given value
and log corresponding error message (it is done this way since eio saves
errno to internal structure so we have to restore it manually). Before
this patch, unlink() call was "in-place" of argument. However, the order
of argument evaluation is unspecified. So it may turn out that we assign
errno to the previous value, which is obviously wrong. To fix it let's
firstly invoke unlink() and then pass the result of call to
xdir_say_gc().