LU-10131 llite: Update i_nlink on unlink

LU-10131 llite: Update i_nlink on unlink

Currently, the client inode link count is not updated on
last unlink.  This is fine because the dentries are all
gone and the inode is eligible for reclaim, but it's still
incorrect.  This causes two problems:

1. Inode is not immediately reclaimed
2. i_nlink count is > 0 for a fully unlinked file, which
confuses wrapfs

On last unlink, the MDT sends back attributes.  Use the
nlink count from these to update the client inode.

Remove null check inherited from ll_get_child_fid, because
the inode should never be null on an unlink.

Re-enabled test 76, which passes with this patch.
Removed slab allocator tuning from test_76, because slab is
no longer the default Linux allocator.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: Ib253b5cf3d35188554cf8fc33a8a3d4b8bb237e8
Reviewed-on: https://review.whamcloud.com/29651
Tested-by: Jenkins
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>