linux: libzfs: mount: fix uninitialised flags

They're later |=d with constants, but never reset

Caught by valgrind while investigating
https://github.com/openzfs/zfs/pull/12928#issuecomment-1007496550

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12954