fs_mgr fix flags/fs_mgr_flags mismatch

Mount flags prefixed with MS_ are intended for the 'flags' entry of
fstab_rec, not fs_mgr_flags.

Coincidentally, this probably has always worked since this maps to the
'wait' flag, which is almost always set for system in the first place.

Test: build
Change-Id: Id4862bc087aa98e1f0e17145dc3ed74b5d867596