OSHMEM: Fix mmap compatibility issue

Using MAP_ANONYMOUS brings different interpretation of fd, offset arguuments of mmap()
Linux:
The mapping is not backed by any file; the fd and offset arguments are ignored.
Mac:
Map anonymous memory not associated with any specific file.  The offset argument is
ignored.  Mac OS X specific: the file descriptor used for creating MAP_ANON regions
can be used to pass some Mach VM flags, and can be specified as -1 if no such flags
are associated with the region
FreeBSD:
Map anonymous memory not associated with any specific file.  The file
descriptor used for creating MAP_ANON must be -1.  The offset   argument must be 0.

fixed by Igor, reviewed by Mike

Refs trac:4399

cmr=v1.7.5:reviewer=ompi-rm1.7

This commit was SVN r31140.

The following Trac tickets were found above:
  Ticket 4399 --> https://svn.open-mpi.org/trac/ompi/ticket/4399

oshmem/mca/sshmem/mmap/sshmem_mmap_module.c

@@ -188,7 +188,7 @@ segment_create(map_segment_t *ds_buf,
                 MAP_ANONYMOUS |
 #endif
                 MAP_FIXED,
-                0,
+                -1,
                 0);
 
     if (MAP_FAILED == addr) {
@@ -243,7 +243,7 @@ segment_attach(map_segment_t *ds_buf, sshmem_mkey_t *mkey)
                 MAP_ANONYMOUS |
 #endif
                 MAP_FIXED,
-                0,
+                -1,
                 0);
 
     if (MAP_FAILED == addr) {