mm/sparse: fix a bad comparison

next_present_section_nr() could only return an unsigned number -1, so
just check it specifically where compilers will convert -1 to unsigned
if needed.

  mm/sparse.c: In function 'sparse_init_nid':
  mm/sparse.c:200:20: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
         ((section_nr >= 0) &&    \
                      ^~
  mm/sparse.c:478:2: note: in expansion of macro
  'for_each_present_section_nr'
    for_each_present_section_nr(pnum_begin, pnum) {
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/sparse.c:200:20: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
         ((section_nr >= 0) &&    \
                      ^~
  mm/sparse.c:497:2: note: in expansion of macro
  'for_each_present_section_nr'
    for_each_present_section_nr(pnum_begin, pnum) {
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/sparse.c: In function 'sparse_init':
  mm/sparse.c:200:20: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
         ((section_nr >= 0) &&    \
                      ^~
  mm/sparse.c:520:2: note: in expansion of macro
  'for_each_present_section_nr'
    for_each_present_section_nr(pnum_begin + 1, pnum_end) {
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~

Link: http://lkml.kernel.org/r/20190228181839.86504-1-cai@lca.pw


Fixes: c4e1be9e ("mm, sparsemem: break out of loops early")
Signed-off-by: Qian Cai <cai@lca.pw>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>