NFS: Use GFP_HIGHUSER for page allocation in nfs_symlink()

nfs_symlink() allocates a GFP_KERNEL page for the pagecache. Most
pagecache pages are allocated using GFP_HIGHUSER, and there's no reason
not to do that in nfs_symlink() as well.

Signed-off-by: Jeff Layton <jlayton@redhat.com>