hugetlb: initialize variable to avoid compiler warning

With the gcc 'maybe-uninitialized' warning enabled, gcc will produce:

  mm/hugetlb.c:6896:20: warning: `chg' may be used uninitialized

This is a false positive, but may be difficult for the compiler to
determine.  maybe-uninitialized is disabled by default, but this gets
flagged as a 0-DAY build regression.

Initialize the variable to silence the warning.

Link: https://lkml.kernel.org/r/20221216224507.106789-1-mike.kravetz@oracle.com


Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>