staging/lustre/llite: changes to avoid cache corruption

ll_find_alias is responsible for getting alias for inode
which can be reused. Directories are assumed to have unique
alias, where in case of non-directories there can be multiple
aliases. In case of lustre there can be two type of aliases
i.e. discon_alias and invalid_alias. Usage of discon_alias in
case of non-directories may corrupt dcache and leads to kernel
crash. Changes made to avoid use of discon_alias in case of
non-directories.

Seagate-bug-id: MRP-2739, MRP-3601
Signed-off-by: Lokesh Nagappa Jaliminche <lokesh.jaliminche@seagate.com>
Reviewed-by: Ujjwal Lanjewar <ujjwal.lanjewar@seagate.com>
Reviewed-by: Ashish Purkar <ashish.purkar@seagate.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@seagate.com>
Tested-by: Parinay Vijayprakash Kondekar <parinay.kondekar@seagate.com>
Reviewed-on: http://review.whamcloud.com/17732
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7613


Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>