Deflake aio_test.

- Most AIO tests call io_setup(nr_events = 128). sizeof(struct io_event)
(128*32 = 4096). However, the actual size of the mapping created by
io_setup() is determined by:

(from fs/aio.c:ioctx_alloc())
/*
 * We keep track of the number of available ringbuffer slots, to prevent
 * overflow (reqs_available), and we also use percpu counters for this.
 *
 * So since up to half the slots might be on other cpu's percpu counters
 * and unavailable, double nr_events so userspace sees what they
 * expected: additionally, we move req_batch slots to/from percpu
 * counters at a time, so make sure that isn't 0:
 */
nr_events = max(nr_events, num_possible_cpus() * 4);
nr_events *= 2;

(from fs/aio.c:aio_setup_ring())
/* Compensate for the ring buffer's head/tail overlap entry */
nr_events += 2; /* 1 is required, 2 for good luck */
size = sizeof(struct aio_ring);
size += sizeof(struct io_event) * nr_events;
nr_pages = PFN_UP(size);

When we mremap() only the first page of a multi-page AIO ring buffer
mapping, fs/aio.c:aio_ring_mremap() updates struct kioctx::mmap_base -
but struct kioctx::mmap_size is untouched, so sys_io_destroy() =>
kill_ioctx() vm_unmaps() the mremapped page, plus some number of pages
after it. Just get the actual size of the mapping from /proc/self/maps.

- Delete test case MremapOver; while it is correct that Linux will not
complain if you overwrite the AIO ring buffer with another mapping, it
won't actually work in the sense that AIO events will not be written to
the new mapping, because Linux stores the struct pages of the ring
buffer in struct kioctx::ring_pages and writes to those through kmap()
rather than using userspace addresses.

- Don't munmap() after mremap(MREMAP_FIXED) returns EFAULT; see new
comment in factored-out test case MremapExpansion.

PiperOrigin-RevId: 267482903

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