mm: Change mmap do_brk_flags() to populate the existing VMA

mm: Change mmap do_brk_flags() to populate the existing VMA

do_brk_flags has already located the modified VMA, so just populate the
region.  Add mm_populate_vma() to mm/gup.c and call the new function
which ignores errors.

Benckmarks in KVM before:
$ ./mmtests/work/sources/wis-0-installed/brk1_threads
testcase:brk increase/decrease of one page
warmup
min:886150 max:886150 total:886150
min:901488 max:901488 total:901488
min:897594 max:897594 total:897594
min:897422 max:897422 total:897422
min:900700 max:900700 total:900700
min:900714 max:900714 total:900714
measurement
min:903016 max:903016 total:903016
min:901224 max:901224 total:901224
min:902626 max:902626 total:902626
min:900518 max:900518 total:900518

Benchmark in KVM after change:
/mmtests/work/sources/wis-0-installed/brk1_threads
testcase:brk increase/decrease of one page
warmup
min:895650 max:895650 total:895650
min:904936 max:904936 total:904936
min:904776 max:904776 total:904776
min:905358 max:905358 total:905358
min:908792 max:908792 total:908792
min:907648 max:907648 total:907648
measurement
min:904368 max:904368 total:904368
min:903936 max:903936 total:903936
min:904720 max:904720 total:904720
min:906626 max:906626 total:906626

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>