blk/kernel: Fix error code mapping in KernelDevice::read.

blk/kernel: Fix error code mapping in KernelDevice::read.

pread returns -1 upon error and stores the error code in errno, and thus
the wrong error was being passed into is_expected_ioerr. This is handled
correctly just a few lines down where we return -errno, so it was likely
just an oversight when adapting this logic from the aio codepath, where
the return code is indeed the errno.

This logic has been incorrect since it was introduced in 2018 via
a1e0ece7f987c7a563b25ec0d02fc6f8445ef54e.

Signed-off-by: Joshua Baergen <jbaergen@digitalocean.com>