fs_mgr: fix checking for casefold feature already enabled

EXT4_FEATURE_INCOMPAT_CASEFOLD is a flag in s_feature_incompat, not in
s_feature_ro_compat.

Bug: 138322712
Test: Treehugger
Change-Id: I07bb1d2b818d423196d8ceebda8900a8adbb19e9
1 file changed