trusty: tipc_fuzzer: Remove use of PAGE_SIZE

tipc_fuzzer writes random data of length 'size' and then
attempts to read back data up to 'TIPC_MAX_MSG_SIZE' in length.

Since 'size' is unrestricted we can always write more than the
subsequent read; and in effect the fuzzer only checks that we
can read something.

Remove the read buffer's dependency on PAGE_SIZE.

Bug: 294914413
Test: mma
Change-Id: I909b49fda4e6ebf49f69b3c09ac29ff8629215ef
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>