btrfs: Remove unnecessary casts in btrfs_read_root_item

There is a messy cast here:
	min_t(int, len, (int)sizeof(*item)));

min_t() should normally cast to unsigned.  It's not possible for "len"
to be negative, but if it were then we definitely wouldn't want to pass
negatives to read_extent_buffer().  Also there is an extra cast.

This patch shouldn't affect runtime, it's just a clean up.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David Sterba <dsterba@suse.com>