btrfs: fix possible deadlock by clearing __GFP_FS flag

Using the GFP_HIGHUSER_MOVABLE flag to allocate the metadata's page may cause
deadlock.
  Task1
  open()
    ...
    btrfs_search_slot()
      ...
      btrfs_cow_block()
	...
	alloc_page()
	  wait for reclaiming
					shrink_slab()
					  ...
					  shrink_icache_memory()
					    ...
					    btrfs_evict_inode()
					      ...
					      btrfs_search_slot()

If the path is locked by task1, the deadlock happens.

So the btree's page cache is different with the file's page cache, it can not
allocate pages by GFP_HIGHUSER_MOVABLE flag, we must clear __GFP_FS flag in
GFP_HIGHUSER_MOVABLE flag.

Reported-by: Itaru Kitayama <kitayama@cl.bb4u.ne.jp>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 04babaf..0627418 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -2536,6 +2536,8 @@
 	BTRFS_I(inode)->flags = btrfs_inode_flags(leaf, inode_item);
 
 	alloc_group_block = btrfs_inode_block_group(leaf, inode_item);
+	if (location.objectid == BTRFS_FREE_SPACE_OBJECTID)
+		inode->i_mapping->flags &= ~__GFP_FS;
 
 	/*
 	 * try to precache a NULL acl entry for files that don't have
@@ -4084,7 +4086,6 @@
 		BTRFS_I(inode)->root = root;
 		memcpy(&BTRFS_I(inode)->location, location, sizeof(*location));
 		btrfs_read_locked_inode(inode);
-
 		inode_tree_add(inode);
 		unlock_new_inode(inode);
 		if (new)