Fix race in case of device destruction.

During device destruction it is possible that open() succeed, but
fdevname() return NULL, that can't be assigned to string variable.
Fix that by adding explicit NULL check.

Also while there switch from fdevname() to fdevname_r().

Sponsored by: iXsystems, Inc.
MFC after: 2 weeks

(cherry picked from commit e49d3eb40324eaffaa13b93f2c4173dfa04dfa34)