gdb/linux-tdep: make linux_find_memory_region_ftype a function_view

Make it a function_view and eliminate the `void *` callback data.
Change the callback functions to lambda functions where they are used.

Change-Id: I239b3650783fa06320f85c2a63273346af81344f
Approved-By: Tom Tromey <tom@tromey.com>