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>
reiser4: block_alloc: improve error handling in reiser4_grab_reserved().

In case of error in reiser4_grab_space(), return the original error code,
not -ENOSPC unconditionally.

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>
reiser4: block_alloc: sanitize grab_enabled modifications.

- check and modify ctx->grab_enabled in reiser4_grab_space(), not reiser4_grab()
- do not re-enable grab before doing second attempt in BA_CAN_COMMIT sequence
  (it is unneeded given the first change)

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>
reiser4: do not mess with grab_enabled; instead, use BA_FORCE.

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>