btrfs: Remove fs_info from alloc_reserved_file_extent

fs_info can be referenced from the transaction handle, which is always
valid. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>