btrfs: declare rsv_count as unsigned int instead of int

rsv_count ultimately gets passed to start_transaction() which
now takes an unsigned int as its num_items parameter.
The value of rsv_count should always be positive so declare it
as being unsigned.

Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Alexandru Moise <00moses.alexander00@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>