anaconda: enable discard option for dom0 filesystems by default

This may have performance impact on some older SSD, but on the other
hand, without this option it's pretty easy to fill the whole LVM thin
pool even if there is plenty free space in dom0.
Note that this doesn't enable it on LUKS layer, this is still disabled
by default.

Fixes QubesOS/qubes-issues#3226

anaconda/pyanaconda/kickstart.py

@@ -885,6 +885,10 @@ def execute(self, storage, ksdata, instClass):
             self.mountpoint = ""
             ty = None
 
+        if self.mountpoint.startswith('/') and not self.fsopts:
+            # enable discard for normal filesystems in dom0
+            self.fsopts = "defaults,discard"
+
         # Sanity check mountpoint
         if self.mountpoint != "" and self.mountpoint[0] != '/':
             raise KickstartParseError(formatErrorMsg(self.lineno,
@@ -1245,6 +1249,10 @@ def execute(self, storage, ksdata, instClass):
             else:
                 ty = storage.default_fstype
 
+        if self.mountpoint.startswith('/') and not self.fsopts:
+            # enable discard for normal filesystems in dom0
+            self.fsopts = "defaults,discard"
+
         if not size and self.size:
             try:
                 size = Size("%d MiB" % self.size)
@@ -1490,6 +1498,10 @@ def execute(self, storage, ksdata, instClass):
             else:
                 ty = storage.default_fstype
 
+        if self.mountpoint.startswith('/') and not self.fsopts:
+            # enable discard for normal filesystems in dom0
+            self.fsopts = "defaults,discard"
+
         # Sanity check mountpoint
         if self.mountpoint != "" and self.mountpoint[0] != '/':
             raise KickstartParseError(formatErrorMsg(self.lineno,