LU-11403 llite: ll_fault fixes

LU-11403 llite: ll_fault fixes

Various error conditions in the fault path can cause us to
not return a page in vm_fault.  Check if it's present
before accessing it.

Additionally, it's not valid to return VM_FAULT_NOPAGE for
page faults.  The correct return when accessing a page that
does not exist is VM_FAULT_SIGBUS.  Correcting this avoids
looping infinitely in the testcase.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I53fc16d91462ac5d4555855dfa067d7fd6716c90
Reviewed-on: https://review.whamcloud.com/34247
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>