bcachefs: set fgf order hint before starting a buffered write

Set the preferred folio order in the fgp_flags by calling
fgf_set_order(). Page cache will try to allocate large folio of the
preferred order whenever possible instead of allocating multiple 0 order
folios.

This improves the buffered write performance up to 1.25x with default
mount options and up to 1.57x when mounted with no_data_io option with
the following fio workload:

fio --name=bcachefs --filename=/mnt/test  --size=100G \
     --ioengine=io_uring --iodepth=16 --rw=write --bs=128k
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io-buffered.c

@@ -677,7 +677,8 @@ int bch2_write_begin(struct file *file, struct address_space *mapping,
 
 	bch2_pagecache_add_get(inode);
 
-	folio = __filemap_get_folio(mapping, pos >> PAGE_SHIFT, FGP_WRITEBEGIN,
+	folio = __filemap_get_folio(mapping, pos >> PAGE_SHIFT,
+				    FGP_WRITEBEGIN | fgf_set_order(len),
 				    mapping_gfp_mask(mapping));
 	if (IS_ERR_OR_NULL(folio))
 		goto err_unlock;
@@ -819,7 +820,7 @@ static int __bch2_buffered_write(struct bch_inode_info *inode,
 	darray_init(&fs);
 
 	ret = bch2_filemap_get_contig_folios_d(mapping, pos, end,
-					       FGP_WRITEBEGIN,
+					       FGP_WRITEBEGIN | fgf_set_order(len),
 					       mapping_gfp_mask(mapping), &fs);
 	if (ret)
 		goto out;