mm/mmap: Change do_brk_flags() to expand existing VMA when possible.

mm/mmap: Change do_brk_flags() to expand existing VMA when possible.

Avoid allocating a new VMA when it is not necessary.  Expand the
existing VMA instead.  This avoids unnecessary tree manipulations.

kvm benchmark before
$ ./mmtests/work/sources/wis-0-installed/brk1_threads
testcase:brk increase/decrease of one page
warmup
min:561754 max:561754 total:561754
min:578322 max:578322 total:578322
min:581282 max:581282 total:581282
min:572182 max:572182 total:572182
min:572572 max:572572 total:572572
min:580408 max:580408 total:580408
measurement
min:578786 max:578786 total:578786
min:574780 max:574780 total:574780
min:577166 max:577166 total:577166
min:581240 max:581240 total:581240
min:580700 max:580700 total:580700
min:584612 max:584612 total:584612

benchmark after
$ ./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
min:900630 max:900630 total:900630
min:901432 max:901432 total:901432

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