LU-12400 llite: Use the new vm_fault_t type

LU-12400 llite: Use the new vm_fault_t type

Linux 4.17 created the new vm_fault_t type

Linux-commit: 1c8f422059ae5da07db7406ab916203f9417e396

Linux 5.1 changed the vm_fault_t type to bitwise unsigned int
which changes the interfaces registered to struct vm_operations_struct

Linux-commit: 3d3539018d2cbd12e5af4a132636ee7fd8d43ef0

Prefer to match the upstream API and fallback to 'int'
where vm_fault_t is not available.

This patch is back-ported from the following one:
Lustre-commit: f2b224a48cb00f885b9df2cc56e349dae5f27f9e
Lustre-change: https://review.whamcloud.com/35500

Test-Parameters: trivial
Cray-bug-id: LUS-7600
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I7122fb0d4af3ee9a19c1a5d0b77c4f13f6850181
Reviewed-on: https://review.whamcloud.com/38478
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>