LU-5179 libcfs: do not leak mm_struct

cfs_get_environ might leak an mm_struct if LU-1735 race condition
hits.
Make sure to mmput in such a case.

Change-Id: I49499c14027358961743d8928ab2edbe54bf72f7
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>