drivers/staging/lustre/lustre/ptlrpc/sec_bulk.c: rename PTRS_PER_PAGE

It duplicates the definition in arch/alpha/include/asm/pgtable.h and emits
warnings.

Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>