ioengines: don't call munmap unless size boundary is exceeded

ioengines: don't call munmap unless size boundary is exceeded

In ioengines using mmap() and munmap(), don't call munmap() when doing an
IO that accesses the last byte of a file. This affects the dev-dax,
libpmem, and mmap ioengines.

Example:
io       12987 DEBUG fio_libpmem_prep
io       12987  io_u->offset 7516061696 : fdd->libpmem_off 0 : io_u->buflen 131072 : fdd->libpmem_sz 7516192768
io       12987 munmap addr=0x7f5600000000 size=0x1c0000000

That accesses the last 128 KiB of a 7 GiB file, triggering an munmap
because the comparison to 7 GiB fails:
	7516061696 + 131072 < 7516192768

Change < to <= so that doesn't occur unless an overflow really
happens.

In one test in linux, this increases performance from 5 GB/s to 20 GB/s
as fio stops spending time in mmap() page table mapping spinlocks and
instead spends its time in memmove().  The perf top results improve from:
  36.81%  [kernel]             [k] queued_spin_lock_slowpath
  24.26%  libc-2.26.so         [.] __memmove_avx_unaligned_erms
   4.09%  [kernel]             [k] __radix_tree_lookup
   3.85%  [kernel]             [k] find_next_iomem_res
   3.70%  [kernel]             [k] _raw_spin_lock_irq
   3.48%  [kernel]             [k] down_read

to:
  79.76%  libc-2.26.so         [.] __memmove_avx_unaligned_erms
   3.23%  libc-2.26.so         [.] vfprintf

Signed-off-by: Jens Axboe <axboe@kernel.dk>