inotify: add do_inotify_init() helper; remove in-kernel call to syscall

inotify: add do_inotify_init() helper; remove in-kernel call to syscall

Using the inotify-internal do_inotify_init() helper allows us to get rid
of the in-kernel call to sys_inotify_init1() 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]

Acked-by: Jan Kara <[email protected]>
Cc: Amir Goldstein <[email protected]>
Cc: [email protected]
Signed-off-by: Dominik Brodowski <[email protected]>