btrfs: use min_size variable to setup block rsv in btrfs_replace_file_extents()

btrfs: use min_size variable to setup block rsv in btrfs_replace_file_extents()

There's no need to calculate again the size for the temporary block
reserve in btrfs_replace_file_extents() - we have already calculated it
and stored it in the 'min_size' variable.

So use the variable to make it more clear and also make the variable const
since it's not supposed to change during the whole function.

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