sysctl.d: set kernel.pid_max=4194304 on 64-bit systems

sysctl.d: set kernel.pid_max=4194304 on 64-bit systems

For CONFIG_BASE_FULL Linux kernels, the maximum number of proc/thread
IDs is set to 32768 by default. This default limit can be quite easily
hit during recovery on nodes with high OSD counts.
To avoid hitting the pid_max default limit, attempt to configure it to
4194304, which corresponds to the maximum limit possible on 64-bit
CONFIG_BASE_FULL kernels.

Fixes: http://tracker.ceph.com/issues/21929
Signed-off-by: David Disseldorp <ddiss@suse.de>