restore splice and sendfile support on kernfs

[Christoph Hellwig]

Hi Greg and Tejun,

this fixes a regression in Linux 5.10 that stopped sendfile and splice
from working on kernfs/sysfs files.

Diffstt:
 file.c |   65 ++++++++++++++++++++++++-----------------------------------------
 1 file changed, 24 insertions(+), 41 deletions(-)