drm: Fix use-after-free read in drm_getunique()

In the Linux kernel, the following vulnerability has been resolved:

drm: Fix use-after-free read in drm_getunique()

There is a time-of-check-to-time-of-use error in drm_getunique() due

to retrieving file_priv->master prior to locking the device's master

mutex.

An example can be seen in the crash report of the use-after-free error

found by Syzbot:

https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803

In the report, the master pointer was used after being freed. This is

because another process had acquired the device's master mutex in

drm_setmaster_ioctl(), then overwrote fpriv->master in

drm_new_set_master(). The old value of fpriv->master was subsequently

freed before the mutex was unlocked.

To fix this, we lock the device's master mutex before retrieving the

pointer from from fpriv->master. This patch passes the Syzbot

reproducer test.