staging/lustre: Revert "LU-2139 osc: Track and limit "unstable" pages"

This seems to be causing multiple issues: LU-3274, LU-3277

[The original commit is folded in the large Lusre patch. So we don't
have an exact commit to revert for kernel client -- Peng Tao]
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>