storage write call path optimizations (#11805)

- write in o_dsync instead of o_direct for smaller
  objects to avoid unaligned double Write() situations
  that may arise for smaller objects < 128KiB
- avoid fallocate() as its not useful since we do not
  use Append() semantics anymore, fallocate is not useful
  for streaming I/O we can save on a syscall
- createFile() doesn't need to validate `bucket` name
  with a Lstat() call since createFile() is only used
  to write at `minioTmpBucket`
- use io.Copy() when writing unAligned writes to allow
  usage of ReadFrom() from *os.File providing zero
  buffer writes().

Makefile

@@ -38,7 +38,7 @@ lint:
 check: test
 test: verifiers build
 	@echo "Running unit tests"
-	@GO111MODULE=on CGO_ENABLED=0 go test -tags kqueue ./... 1>/dev/null
+	@GOGC=25 GO111MODULE=on CGO_ENABLED=0 go test -tags kqueue ./... 1>/dev/null
 
 test-race: verifiers build
 	@echo "Running unit tests under -race"