Linux: adapt to truncate sequence changes

Linux: adapt to truncate sequence changes

As part of changes to the truncate sequence, inode_setattr() no
longer exists, and all filesystems have to define the setattr op
so we can assume that it is not NULL.

Introduce a compat inline function afs_inode_setattr that hides
the tests and the different versions from the main code.

Note that the existing test for the inode_setattr() return type
will fail, but the value is no longer used in that case.

This is required for 2.6.36

Change-Id: I2f5e8a0b660b48453d0152b6c4db64e57539f91a
Reviewed-on: http://gerrit.openafs.org/2543
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>