libibverbs: Fall back to the write ABI in ibv_cmd_reg_mr_ex()

ibv_cmd_reg_mr_ex() issued the REG_MR ioctl unconditionally (execute_ioctl)
with no fallback to the legacy write-based registration, unlike
ibv_cmd_reg_mr() which is write-ABI based.  An address-based MR registered
through the extended path therefore fails on a kernel old enough to lack
the REG_MR ioctl method, even though it still supports the legacy write ABI
that ibv_cmd_reg_mr() uses.

Issue the command through the existing ioctl/write fallback machinery
instead: declare the command buffer with DECLARE_FBCMD_BUFFER and dispatch
via execute_ioctl_fallback().  On TRY_WRITE -- the kernel has no REG_MR
ioctl method -- fall back to the legacy write REG_MR command via
ibv_cmd_reg_mr() for the address-based case.  A dma-buf (fd) or DMA-handle
request has no write-ABI equivalent, and a kernel without the ioctl could
not service it anyway, so those return EOPNOTSUPP.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>