x86/kaslr: Small cleanup of find_random_phys_addr()

Just a trivial rearrangement to do all the processing together, and only
have one call to slots_fetch_random() in the source.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200728225722.67457-18-nivedita@alum.mit.edu