Use a proper allocator for rmap entries using a naive for loop. The
allocator relies on whether an entry is NULL to be considered free.
Remove the used field of rmap which is not needed.

Also, avoid crashing the kernel if an entry is not available.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NEli Cohen <elic@nvidia.com>
Signed-off-by: NSaeed Mahameed <saeedm@nvidia.com>
Reviewed-by: NJacob Keller <jacob.e.keller@intel.com>