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().

buildscripts/verify-build.sh

@@ -33,6 +33,7 @@ export ACCESS_KEY="minio"
 export SECRET_KEY="minio123"
 export ENABLE_HTTPS=0
 export GO111MODULE=on
+export GOGC=25
 
 MINIO_CONFIG_DIR="$WORK_DIR/.minio"
 MINIO=( "$PWD/minio" --config-dir "$MINIO_CONFIG_DIR" )