doc: add dma-buf IOCTL code to table

The code 'b' is used for dma-buf IOCTLs.
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221128133853.355645-1-contact@emersion.fr

Documentation/userspace-api/ioctl/ioctl-number.rst

@@ -221,6 +221,7 @@ Code  Seq#    Include File                                           Comments
 'a'   00-0F  drivers/crypto/qat/qat_common/adf_cfg_common.h          conflict! qat driver
 'b'   00-FF                                                          conflict! bit3 vme host bridge
                                                                      <mailto:natalia@nikhefk.nikhef.nl>
+'b'   00-0F  linux/dma-buf.h                                         conflict!
 'c'   all    linux/cm4000_cs.h                                       conflict!
 'c'   00-7F  linux/comstats.h                                        conflict!
 'c'   00-7F  linux/coda.h                                            conflict!