exiting the entire bash script from the subshell

In general, you can use kill "$$" from a subshell in order to terminate the main script ($$ will expand to the pid of the main shell even in subshells, and you can set a TERM trap in order to catch that signal).

But it looks that you actually want to terminate the left side of a pipeline from its right side — i.e. cause inotifywait to terminate without waiting until it’s writing something to the orphan pipe and is killed by SIGPIPE. For that you can kill just the inotifywait process explicitly with pkill:

inotifywait -m /some/dir -e create,modify |
while read path action file; do
   pkill -PIPE -P "$$" -x inotifywait
done

pkill -P selects by parent; $$ should be the PID of your script. This solution is of course, not fool-proof. Also have a look at this.