Fix an off-by-one in locate_hole()

Fix an off-by-one in locate_hole() that can cause it to return a range
that was previously allocated.

In upgrading to kexec-tools 2.0.5 I first got the error "Overlapping
memory segments at 0xbeff000"

Adding some debugging I found locate_hole was returning incorrect
values.  The below is from the debug I added:

XXXMDF: look for hole size 100000, cur range [52b3000, bffffff] size 6d4cfff
XXXMDF: look for hole memsz=100000, found beff000

Hmm, if we wanted 0x100000 bytes ending at 0xbffffff, that should be
0xbf00000, not 0xbef000.  Continuing to the second invocation:

XXXMDF: look for hole size 1000, cur range [52b3000, befefff] size 6c4bfff
XXXMDF: look for hole size 1000, cur range [bfff000, bffffff] size fff
XXXMDF: look for hole memsz=1000, found bffe000

Now we die with overlapping ranges, since the 0x100000 bytes at
0xbeff000 overlaps 0x1000 bytes at 0xbffe000.

Signed-off-by: Matthew Fleming <mdf356@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>