btrfs: use a negative value for BTRFS_LOG_FORCE_COMMIT

Currently we use the value 1 for BTRFS_LOG_FORCE_COMMIT, but that value
has a few inconveniences:

1) If it's ever used by btrfs_log_inode(), or any function down the call
   chain, we have to remember to btrfs_set_log_full_commit(), which is
   repetitive and has a chance to be forgotten in future use cases.
   btrfs_log_inode_parent() only calls btrfs_set_log_full_commit() when
   it gets a negative value from btrfs_log_inode();

2) Down the call chain of btrfs_log_inode(), we may have functions that
   need to force a log commit, but can return either an error (negative
   value), false (0) or true (1). So they are forced to return some
   random negative to force a log commit - using BTRFS_LOG_FORCE_COMMIT
   would make the intention more clear. Currently the only example is
   flush_dir_items_batch().

So turn BTRFS_LOG_FORCE_COMMIT into a negative value. The chosen value
is -(MAX_ERRNO + 1), so that it does not overlap any errno value and makes
it easier to debug.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>