net/hyperv: Remove unnecessary kmap_atomic in netvsc driver

__get_free_pages() doesn't return HI memory, so the memory is always mapped.
kmap_atomic() is not necessary here. This patch removes the kmap_atomic()
calls and related code for locking and page manipulation.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>