Set stack as function of max call depth

[Alan-Jowett]

This pull request includes changes to the vm/inc/ubpf.h file to update the configuration of the uBPF VM. The most important changes involve setting a default maximum number of nested calls and adjusting the default stack size accordingly.

Configuration updates:

• Added a new definition for UBPF_MAX_CALL_DEPTH with a default value of 8 if not already defined.
• Adjusted the default stack size (UBPF_EBPF_STACK_SIZE) to be calculated based on UBPF_MAX_CALL_DEPTH.
• Removed an older definition for UBPF_MAX_CALL_DEPTH that set the value to 10.

[coveralls]

coverage: 79.201%. remained the same when pulling e594cf286a0ed1c7a207b120a4761a5fb9695c68 on Alan-Jowett:reduce_stack_size into 94f7a4351b200a2e89b7eafa751c5e29efd22dd7 on iovisor:main.