dax: Fix sleep in atomic contex in grab_mapping_entry()

Commit 642261ac995e: "dax: add struct iomap based DAX PMD support" has
introduced unmapping of page tables if huge page needs to be split in
grab_mapping_entry(). However the unmapping happens after
radix_tree_preload() call which disables preemption and thus
unmap_mapping_range() tries to acquire i_mmap_lock in atomic context
which is a bug. Fix the problem by moving unmapping before
radix_tree_preload() call.

Fixes: 642261ac995e01d7837db1f4b90181496f7e6835
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/dax.c

@@ -333,14 +333,6 @@ restart:
 		}
 
 		spin_unlock_irq(&mapping->tree_lock);
-		err = radix_tree_preload(
-				mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM);
-		if (err) {
-			if (pmd_downgrade)
-				put_locked_mapping_entry(mapping, index, entry);
-			return ERR_PTR(err);
-		}
-
 		/*
 		 * Besides huge zero pages the only other thing that gets
 		 * downgraded are empty entries which don't need to be
@@ -350,6 +342,13 @@ restart:
 			unmap_mapping_range(mapping,
 				(index << PAGE_SHIFT) & PMD_MASK, PMD_SIZE, 0);
 
+		err = radix_tree_preload(
+				mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM);
+		if (err) {
+			if (pmd_downgrade)
+				put_locked_mapping_entry(mapping, index, entry);
+			return ERR_PTR(err);
+		}
 		spin_lock_irq(&mapping->tree_lock);
 
 		if (pmd_downgrade) {