sunrpc: convert unnecessary GFP_ATOMIC to GFP_NOFS

It's OK to sleep here, we just don't want to recurse into the filesystem
as a writeout could be waiting on this.

Future work: the documentation for GFP_NOFS says "Please try to avoid
using this flag directly and instead use memalloc_nofs_{save,restore} to
mark the whole scope which cannot/shouldn't recurse into the FS layer
with a short explanation why. All allocation requests will inherit
GFP_NOFS implicitly."

But I'm not sure where to do this.  Should the workqueue be arranging
that for us in the case of workqueues created with WQ_MEM_RECLAIM?

Reported-by: Trond Myklebust <trondmy@hammer.space>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>