btrfs: move need_log_inode() to above log_conflicting_inodes()

The static function need_log_inode() is defined below btrfs_log_inode()
and log_conflicting_inodes(), but in the next patches in the series we
will need to call need_log_inode() in a couple new functions that will be
used by btrfs_log_inode(). So move its definition to a location above
log_conflicting_inodes().

Also make its arguments 'const', since they are not supposed to be
modified.

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