Linux: Fix write_begin configure test for recent RHEL kernels

Linux: Fix write_begin configure test for recent RHEL kernels

Recent RHEL kernels now define simple_write_begin, which was used as
a test for the write_begin address_space op.  This makes the test
succeed when it shouldn't, and breaks the build.
Rewrite the test to actually check the address_space operation.

Change-Id: Idac9b318ff716b61bf8ca4508d2dbdbfbad5b50d
Reviewed-on: http://gerrit.openafs.org/759
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>