media: media-request: Fix crash if memory allocation fails

Syzbot reports a NULL-ptr deref in the kref_put() call:

BUG: KASAN: null-ptr-deref in media_request_put drivers/media/mc/mc-request.c:81 [inline]
 kref_put include/linux/kref.h:64 [inline]
 media_request_put drivers/media/mc/mc-request.c:81 [inline]
 media_request_close+0x4d/0x170 drivers/media/mc/mc-request.c:89
 __fput+0x2ed/0x750 fs/file_table.c:281
 task_work_run+0x147/0x1d0 kernel/task_work.c:123
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop arch/x86/entry/common.c:165 [inline]
 prepare_exit_to_usermode+0x48e/0x600 arch/x86/entry/common.c:196

What led to this crash was an injected memory allocation failure in
media_request_alloc():

FAULT_INJECTION: forcing a failure.
name failslab, interval 1, probability 0, space 0, times 0
 should_failslab+0x5/0x20
 kmem_cache_alloc_trace+0x57/0x300
 ? anon_inode_getfile+0xe5/0x170
 media_request_alloc+0x339/0x440
 media_device_request_alloc+0x94/0xc0
 media_device_ioctl+0x1fb/0x330
 ? do_vfs_ioctl+0x6ea/0x1a00
 ? media_ioctl+0x101/0x120
 ? __media_device_usb_init+0x430/0x430
 ? media_poll+0x110/0x110
 __se_sys_ioctl+0xf9/0x160
 do_syscall_64+0xf3/0x1b0

When that allocation fails, filp->private_data is left uninitialized
which media_request_close() does not expect and crashes.

To avoid this, reorder media_request_alloc() such that
allocating the struct file happens as the last step thus
media_request_close() will no longer get called for a partially created
media request.

Reported-by:  <syzbot+6bed2d543cf7e48b822b@syzkaller.appspotmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Fixes: 10905d70

 ("media: media-request: implement media requests")
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>