staging/lustre/vvp: release mmap_sem in error case

The mmap_sem is downed in vvp_mmap_locks, but in case of
error from cl_io_lock_alloc_add, it is not upped.

Credit to Paul Casella at Cray for finding this.

Signed-off-by: Patrick Farrell <paf@cray.com>
Reviewed-on: http://review.whamcloud.com/10741
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5221


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>