fuse: fix link error with CONFIG_FUSE_PASSTHROUGH=n

Commit 2f19d54c055d ("fuse: support 64 bit, server allocated backing ID")
moved the CONFIG_FUSE_PASSTHROUGH check out of the backing ioctl handlers
and into fuse_get_conn_for_passthrough(). The calls from the handlers
into backing.c, which is only built with CONFIG_FUSE_PASSTHROUGH, are now
only removed if the compiler inlines that helper. It does not have to,
and with gcc 11 and CONFIG_CC_OPTIMIZE_FOR_SIZE it doesn't:

  dev.c:(.text+0x355c): undefined reference to `fuse_backing_open'
  nios2-linux-ld: dev.c:(.text+0x3594): undefined reference to `fuse_backing_open_64'
  nios2-linux-ld: dev.c:(.text+0x35d0): undefined reference to `fuse_backing_close'

Check CONFIG_FUSE_PASSTHROUGH in each handler again, as
fuse_notify_backing_remove() does. The check still comes after the
fuse_get_dev() and -ENOTCONN checks, so the errors userspace sees do not
change.

Fixes: 2f19d54c055d ("fuse: support 64 bit, server allocated backing ID")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202610171310.uqRzJkIx-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202610260719.sj08mXBr-lkp@intel.com/
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>