s390/mm: Simplify gap clamping in mmap_base() using clamp()

s390/mm: Simplify gap clamping in mmap_base() using clamp()

mmap_base() has logic to ensure that the variable "gap" stays within the
range defined by "gap_min" and "gap_max". Replace this with the clamp()
macro to shorten and simplify code.

Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
Link: https://lore.kernel.org/r/20250204162508.12335-1-qasdev00@gmail.com
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
[gor@linux.ibm.com: also remove the gap_min and gap_max variables]
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>