Change mmap(2) to indicate ENOMEM is returned if the requested area is already mapped

[felix.the.red_gmail.com]

Summary

Change mmap(2) to document that ENOMEM is returned instead of EINVAL in a specific case.

From the original PR:
The man page for mmap(2) says:

[EINVAL]           Both MAP_FIXED and MAP_EXCL were specified, but the
                   requested region is already used by a mapping.

This should be ENOMEM rather than EINVAL.

PR:215469

Test Plan

Attachment with test code

If ENOMEM is returned, the test code will say "Successfully detected clash with ENOMEM" for each successful detection.

[ziaee]

Few points:

• PR goes in the commit message, i.e. the summary section
• Test plan should show how I can verify this

That would make this go a lot faster.