Fix: btrfs: Remove unnecessary fs_info parameter

See upstream commit:

  commit 3dca5c942dac60164e6a6e89172f25b86af07ce7
  Author: Qu Wenruo <wqu@suse.com>
  Date:   Thu Apr 26 14:24:25 2018 +0800

    btrfs: trace: Remove unnecessary fs_info parameter for btrfs__reserve_extent event class

    fs_info can be extracted from btrfs_block_group_cache, and all
    btrfs_block_group_cache is created by btrfs_create_block_group_cache()
    with fs_info initialized, no need to worry about NULL pointer
    dereference.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>