switch do_fsync() to fget_light()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
Signed-off-by: Willi Ye <williye97@gmail.com>

Conflicts:
	fs/sync.c
kthread: squash https://github.com/torvalds/linux/commit/9a2e03d8ed518a61154f18d83d6466628e519f94 and https://github.com/torvalds/linux/commit/46f3d976213452350f9d10b0c2780c2681f7075b

Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>