staging: lustre: libcfs: move memory_pressure functions to libcfs_prim.h

staging: lustre: libcfs: move memory_pressure functions to libcfs_prim.h

Long ago libcfs_prim.h was used for userland code which is why
memory_pressure_*() handling is in both libcfs_prim.h and
linux-mem.h headers. So lets just move the memory_pressure_*()
to libcfs_prim.h.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6245
Reviewed-on: http://review.whamcloud.com/13841
Reviewed-by: frank zago <fzago@cray.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>