malloc: support callbacks on memory events

malloc: support callbacks on memory events

Each process will have its own callbacks. Callbacks will indicate
whether it's allocation and deallocation that's happened, and will
also provide start VA address and length of allocated block.

Since memory hotplug isn't supported on FreeBSD and in legacy mem
mode, it will not be possible to register them in either.

Callbacks are called whenever something happens to the memory map of
current process, therefore at those times memory hotplug subsystem
is write-locked, which leads to deadlocks on attempt to use these
functions. Document the limitation.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Tested-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Tested-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>