skbuff: inline page_frag_alloc_align()

The __alloc_frag_align() is short, and only called by two functions,
so inline page_frag_alloc_align() for reduce the overhead of calls.

Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>