f2fs: skip direct I/O iostat context when disabled

F2FS iostat is optional and is disabled by default.  Direct I/O still
allocates and binds a bio_iostat_ctx, updates the submit timestamp, and
replaces bi_end_io for every DIO bio even when sbi->iostat_enable is
false.

The byte accounting calls do not need an extra guard because
f2fs_update_iostat() already checks sbi->iostat_enable.  Only skip the
DIO bio context setup when iostat is disabled.  If iostat is enabled
through sysfs before submission, the existing context allocation and
latency accounting path is still used.

QEMU benchmark on a 1GiB F2FS virtio-blk image, with iostat_enable=0,
4KiB O_DIRECT I/O over a 64MiB file, 50000 iterations per run:

                         baseline     patched
  direct_read median    65264.50 ns  55470.95 ns
  direct_read recheck   65553.75 ns  55470.95 ns
  direct_write median   68054.62 ns  56309.44 ns
  direct_write recheck  66873.51 ns  56309.44 ns

Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>