xfs: fix might_sleep() warning when initialising per-ag tree

The use of radix_tree_preload() only works if the radix tree was
initialised without the __GFP_WAIT flag. The per-ag tree uses
GFP_NOFS, so does not trigger allocation of new tree nodes from the
preloaded array. Hence it enters the allocator with a spinlock held
and triggers the might_sleep() warnings.

Reported-by; Chris Mason <[e-mail]>
Signed-off-by: Dave Chinner <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Alex Elder <[e-mail]>

fs/xfs/xfs_mount.c

···
 	 * Allocate and initialize the per-ag data.
 	 */
 	spin_lock_init(&mp->m_perag_lock);
-	INIT_RADIX_TREE(&mp->m_perag_tree, GFP_NOFS);
+	INIT_RADIX_TREE(&mp->m_perag_tree, GFP_ATOMIC);
 	error = xfs_initialize_perag(mp, sbp->sb_agcount, &mp->m_maxagi);
 	if (error) {
 		cmn_err(CE_WARN, "XFS: Failed per-ag init: %d", error);