btrfs: ignore incorrect btrfs_file_extent_item::ram_bytes

[HICCUP]
Kernels can create file extent items with incorrect ram_bytes like this:

	item 6 key (257 EXTENT_DATA 0) itemoff 15816 itemsize 53
		generation 7 type 1 (regular)
		extent data disk byte 13631488 nr 32768
		extent data offset 0 nr 4096 ram 4096
		extent compression 0 (none)

Thankfully kernel can handle them properly, as in that case ram_bytes is
not utilized at all.

[ENHANCEMENT]
Since the hiccup is not going to cause any data-loss and is only a minor
violation of on-disk format, here we only need to ignore the incorrect
ram_bytes value, and use the correct one from
btrfs_file_extent_item::disk_num_bytes.

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