drm: add a locked version of drm_is_current_master

While checking the master status of the DRM file in
drm_is_current_master(), the device's master mutex should be
held. Without the mutex, the pointer fpriv->master may be freed
concurrently by another process calling drm_setmaster_ioctl(). This
could lead to use-after-free errors when the pointer is subsequently
dereferenced in drm_lease_owner().

The callers of drm_is_current_master() from drm_auth.c hold the
device's master mutex, but external callers do not. Hence, we implement
drm_is_current_master_locked() to be used within drm_auth.c, and
modify drm_is_current_master() to grab the device's master mutex
before checking the master status.

Reported-by: Daniel Vetter <[email protected]>
Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
Reviewed-by: Emil Velikov <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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