dmaengine: idxd: Fix a possible NULL pointer dereference

mainline inclusion
from mainline-v5.15
commit e9c5b0b5
category: bugfix
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I596WO


CVE: NA

Intel-SIG: commit e9c5b0b5 dmaengine: idxd: Fix a possible NULL pointer dereference.
Incremental backporting patches for DSA/IAA on Intel Xeon platform.

--------------------------------

'device_driver_attach()' dereferences its first argument (i.e. 'alt_drv')
so it must not be NULL.
Simplify the error handling logic about NULL 'alt_drv' in order to be
more robust and future-proof.

Fixes: 568b2126 ("dmaengine: idxd: fix uninit var for alt_drv")
Fixes: 6e7f3ee9 ("dmaengine: idxd: move dsa_drv support to compatible mode")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/77f0dc4f3966591d1f0cffb614a94085f8895a85.1627560174.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Xiaochen Shen <xiaochen.shen@intel.com>