fs: add kern_select() helper; remove in-kernel call to sys_select()

fs: add kern_select() helper; remove in-kernel call to sys_select()

Using this helper allows us to avoid the in-kernel call to the sys_umount()
syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527[email protected]

Cc: Al Viro <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Dominik Brodowski <[email protected]>