LU-2753 lvfs: cleanup lvfs.h and collateral

LU-2753 lvfs: cleanup lvfs.h and collateral

Remove the unused struct lvfs_ucred. Remove the ucred pointer argument
from {push,pop}_ctxt() to which all called passed NULL. Remove the now
unused functions {push,pop}_group_info(). Delete lvfs_linux.h. Remove
the cb_ops member from lvfs_run_ctxt which was read but never set.
Remove the unused functions {l,ll}_dentry_open(). Reduce the number
of gratuitous includes of lvfs.h.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: Ia7f61e4c5a6af73381a39740dc76367655d18985
Reviewed-on: http://review.whamcloud.com/6660
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>