fs/dax: Fix pmd vs pte conflict detection

Users reported a v5.3 performance regression and inability to establish
huge page mappings. A revised version of the ndctl "dax.sh" huge page
unit test identifies commit 23c84eb7 "dax: Fix missed wakeup with
PMD faults" as the source.

Update get_unlocked_entry() to check for NULL entries before checking
the entry order, otherwise NULL is misinterpreted as a present pte
conflict. The 'order' check needs to happen before the locked check as
an unlocked entry at the wrong order must fallback to lookup the correct
order.
Reported-by: NJeff Smits <jeff.smits@intel.com>
Reported-by: NDoug Nelson <doug.nelson@intel.com>
Cc: <stable@vger.kernel.org>
Fixes: 23c84eb7 ("dax: Fix missed wakeup with PMD faults")
Reviewed-by: NJan Kara <jack@suse.cz>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Link: https://lore.kernel.org/r/157167532455.3945484.11971474077040503994.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: NDan Williams <dan.j.williams@intel.com>

fs/dax.c

@@ -220,10 +220,11 @@ static void *get_unlocked_entry(struct xa_state *xas, unsigned int order)
 
 	for (;;) {
 		entry = xas_find_conflict(xas);
+		if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
+			return entry;
 		if (dax_entry_order(entry) < order)
 			return XA_RETRY_ENTRY;
-		if (!entry || WARN_ON_ONCE(!xa_is_value(entry)) ||
-				!dax_is_locked(entry))
+		if (!dax_is_locked(entry))
 			return entry;
 
 		wq = dax_entry_waitqueue(xas, entry, &ewait.key);