Return ENOTSUP for Linux FS_IOC_FIEMAP ioctl.

Linux man(1) calls it for no good reason; this avoids the console spam
(eg '(man): ioctl fd=4, cmd=0x660b ('f',11) is not implemented').

Reviewed by: emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20690