video: tegra: nvmap: fix cache_maint leak

Under certain error conditions, nvmap's cache_maint would do an early
return while holding an incremented refcount and an allocated pte,
causing leaks.

Bug 1042888

Change-Id: Ie6d557f5678e7475cf46c1524a43ed464502e94c
Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Reviewed-on: http://git-master/r/131887
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>