Update mBpfMap name for clarity

This change updates 'mBpfMap' to 'mIndexToIfaceBpfMap' to
better-describe what the bpf map contains. This is done as a
prerequisite change for adding a name->indices map.

Test: Build
Bug: 241098920
Change-Id: Idaab7072a14ea3c88e9a5df808e768f355d83782