btrfs: avoid used space computation when reserving space

In __reserve_bytes() we have 3 repeated calls to btrfs_space_info_used(),
one early on as soon as take the space_info's spinlock, another one when
we call btrfs_can_overcommit(), which calls btrfs_space_info_used() again,
and a final one when we are reserving for a flush emergency.

During all these calls we are holding the space_info's spinlock, which is
heavily used by the space reservation and flushing code, so it's desirable
to make the critical sections as short as possible.

So make this more efficient by:

1) Instead of calling btrfs_can_overcommit() call the new variant
   can_overcommit() which takes the space_info's used space as an argument
   and pass the value we already computed and have in the 'used' variable;

2) Instead of calling btrfs_space_info_used() with its second argument as
   false when we are doing a flush emergency, decrement the space_info's
   bytes_may_use counter from the 'used' variable, as the difference
   between passing true or false as the second argument to
   btrfs_space_info_used() is whether or not to include the space_info's
   bytes_may_use counter in the computation.

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

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