UKERNEL: f_fsid is a struct on AIX

UKERNEL: f_fsid is a struct on AIX

The statvfs f_fsid member is a struct with two fields on AIX when
_ALL_SOURCE is defined (which we define to get various extensions). So
in the AIX case, use it as such a struct instead of a single integer.

Change-Id: Ibd94d6b38b0cdc08b5812fe29e4e93344765ff05
Reviewed-on: http://gerrit.openafs.org/3260
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>