fs/nfs: Replace kmap_atomic() with kmap_local_page() in dir.c

kmap_atomic() is deprecated in favor of kmap_local_page().

With kmap_local_page() the mappings are per thread, CPU local, can take
page-faults, and can be called from any context (including interrupts).
Furthermore, the tasks can be preempted and, when they are scheduled to
run again, the kernel virtual addresses are restored and still valid.

kmap_atomic() is implemented like a kmap_local_page() which also disables
page-faults and preemption (the latter only for !PREEMPT_RT kernels,
otherwise it only disables migration).

The code within the mappings/un-mappings in the functions of dir.c don't
depend on the above-mentioned side effects of kmap_atomic(), so that mere
replacements of the old API with the new one is all that is required
(i.e., there is no need to explicitly add calls to pagefault_disable()
and/or preempt_disable()).

Therefore, replace kmap_atomic() with kmap_local_page() in fs/nfs/dir.c.

Tested in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
HIGHMEM64GB enabled.

Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>