KVM: selftests: Use EPOLL in userfaultfd_util reader threads

With multiple reader threads POLLing a single UFFD, the demand paging test
suffers from the thundering herd problem: performance degrades as the
number of reader threads is increased. Solve this issue [1] by switching
the the polling mechanism to EPOLL + EPOLLEXCLUSIVE.

Also, change the error-handling convention of uffd_handler_thread_fn.
Instead of just printing errors and returning early from the polling
loop, check for them via TEST_ASSERT(). "return NULL" is reserved for a
successful exit from uffd_handler_thread_fn, i.e. one triggered by a
write to the exit pipe.

Performance samples generated by the command in [2] are given below.

Num Reader Threads, Paging Rate (POLL), Paging Rate (EPOLL)
1      249k      185k
2      201k      235k
4      186k      155k
16     150k      217k
32     89k       198k

[1] Single-vCPU performance does suffer somewhat.
[2] ./demand_paging_test -u MINOR -s shmem -v 4 -o -r <num readers>

Signed-off-by: Anish Moorthy <amoorthy@google.com>
Acked-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20240215235405.368539-13-amoorthy@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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