Linux 4.5: get_link instead of follow_link+put_link

Linux 4.5: get_link instead of follow_link+put_link

In linux commit 6b255391, the follow_link inode operation was
replaced by the get_link operation, which is basically the same
but takes the inode and dentry separately, allowing for the
possibility of staying in RCU mode.

For now, only support this if page_get_link is available and we are
using the USABLE_KERNEL_PAGE_SYMLINK_CACHE

The previous test for USABLE_KERNEL_PAGE_SYMLINK_CACHE used a bogus,
undefined configure variable (ac_cv_linux_kernel_page_follow_link).
Remove it, as it was not needed

Reviewed-on: https://gerrit.openafs.org/12265
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Joe Gorse <jhgorse@gmail.com>
Tested-by: Joe Gorse <jhgorse@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 2ef27ea1bb032cee8d26980e60e02b52a0805763)

Change-Id: I828823ad16f24bae583de9cf436844565217918d
Reviewed-on: https://gerrit.openafs.org/12301
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>