reiser4: implement ->remount_fs() super operation

Add ->remount_fs() super option, which performs sync_filesystem()
needed when when transitioning from read-write to read-only, see
http://marc.info/?l=samba-technical&m=139550798721412&w=2 for details.

Panic when flushing read-only file system.

Signed-off-by: Edward Shishkin <edward.shishkin@gmail.com>