Deflake mount_test.
Deflake mount_test.

Inode ids are only stable across Save/Restore if we have an open FD on the
inode. All tests that compare inode ids must therefor hold an FD open.

PiperOrigin-RevId: 254086603

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor