file: Simplify fs_info and append_support structs
file: Simplify fs_info and append_support structs

These two combine the information about underlying fs, but
in a bit hairy manner. This info will be shared with files
soon, so deserves some brush-up.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

avikivity/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar