btrfs: subpage: make btrfs_submit_compressed_write() compatible

There is a WARN_ON() checking if @start is aligned to PAGE_SIZE, not
sectorsize, which will cause false alert for subpage.  Fix it to check
against sectorsize.

Furthermore:

- Use ASSERT() to do the check
  So that in the future we may skip the check for production build

- Also check alignment for @len

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.c

@@ -513,7 +513,8 @@ blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
 	const bool use_append = btrfs_use_zone_append(inode, disk_start);
 	const unsigned int bio_op = use_append ? REQ_OP_ZONE_APPEND : REQ_OP_WRITE;
 
-	WARN_ON(!PAGE_ALIGNED(start));
+	ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
+	       IS_ALIGNED(len, fs_info->sectorsize));
 	cb = kmalloc(compressed_bio_size(fs_info, compressed_len), GFP_NOFS);
 	if (!cb)
 		return BLK_STS_RESOURCE;