Provide access to file generation number on Linux.
Add test coverage.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes#12856
