btrfs: use a btrfs_inode in the log context (struct btrfs_log_ctx)

Instead of using a inode pointer, use a btrfs_inode pointer in the log
context structure, as this is generally what we need and allows for some
internal APIs to take a btrfs_inode instead, making them more consistent
with most of the code base. This will later allow to help to remove a lot
of BTRFS_I() calls in btrfs_sync_file().

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>