Currently the memory map is being created
with a size that is greater than the size 
of the underlying datafile. This can cause
varying behaviour,

e.g. 

In windows the size of the datafile
is increased, while on linux it remains
the same.

This fix removes the increment margin to
the size that is used while creating the
memory map.

storage/myisam/mi_dynrec.c:
  remove MEMMAP_EXTRA_MARGIN that is used as
  the increment margin to the underlying 
  datafile size while creating the mmap.
storage/myisam/mi_packrec.c:
  The size of the underlying datafile is
  increased by MEMMAP_EXTRA_MARGIN when
  using a packed record format. Hence in 
  this case the size of the memory map should 
  be incremented by the same factor.