docs: vm: page_frags.rst: Fix the reference of stale function

The __netdev_alloc_frag() has been dropped by commit 7ba7aeab

 ("net:
Don't disable interrupts in napi_alloc_frag()"). So replace it with
netdev_alloc_frag().

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Link: https://lore.kernel.org/r/20200508140924.18220-1-haokexin@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>