LinuxKPI: fix possible NULL dereference in linuxkpi_page_frag_alloc()

Fix a possible NULL pointer deref in case alloc_pages() fails.
This is theoretical so far as up to now no code in the tree uses
linuxkpi_page_frag_alloc().

Reported by: Coverity via emaste
Coverity ID: 1502345
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
X-MFC-with: 55038a6306a570c9f2df89f5ad076de0f7d98152