btrfs: avoid used space computation when trying to grant tickets

btrfs: avoid used space computation when trying to grant tickets

In btrfs_try_granting_tickets(), we call btrfs_can_overcommit() and that
calls btrfs_space_info_used(). But we already keep track, in the 'used'
local variable, of the used space in the space_info, so we are just
repeating the same computation and doing an extra function call while we
are holding the space_info's spinlock, which is heavily used by the space
reservation and flushing code.

So add a local variant of btrfs_can_overcommit() that takes in the used
space as an argument and therefore does not call btrfs_space_info_used(),
and use it in btrfs_try_granting_tickets().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>