BACKPORT: Add support for BPF_FUNC_probe_read_str

Downstream kernel is missing support for this. It is needed by I/O
readahead project to read strings from kernel memory on Pixel v4.9. Add
support for the same.

Bug: 135210418

Change-Id: I37895b395d3fd2637f1d69e111c4a6e2b8c0327a
Signed-off-by: Joel Fernandes <joelaf@google.com>