x86/mm: Replace open-coded gap bounding with clamp()

Rather than manually bounding gap between gap_min and gap_max,
use the well-known clamp() macro to make the code easier to read.

Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250215125249.10729-1-qasdev00@gmail.com