Don't return AOP_WRITEPAGE_ACTIVATE to write()

Don't return AOP_WRITEPAGE_ACTIVATE to write()

When we're called from write(), we don't have the option
of deferring the writing of a page by returning AOP_WRITEPAGE_ACTIVATE.
Instead, write() simply sees this as the output of 0x8000 bytes of data.

So, whilst we can mark a vcache as being output, we can't defer the
processing of one which is already being written (by, for example, an
earlier writepage()).

This problem only affects files which are have mmap() and write()
called in quick succession, but it does break the fsx utility.

Change-Id: I750a186de38da9873665a862f5b584a78e6979ad
Reviewed-on: http://gerrit.openafs.org/725
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>