backport: add no_seek_end_llseek()

no_seek_end_llseek() was added with kernel 4.5 in commit fb1d259e4 "new
helpers: no_seek_end_llseek{,_size}()". This is used by wlcore.
generic_file_llseek_size() is only available in kernel >= 3.2.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>