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.
reiser4: block_alloc: create a separate function for printing a superblock's block counters.
reiser4: block_alloc: add a BA_ALL flag for grabbing all possible space.
reiser4: block_alloc: implement exclusive space grabbing through read-write semaphores.

This is now used to augment the second pass of BA_CAN_COMMIT machinery.
reiser4: block_alloc: sanitize grab_enabled modifications.

- do not re-enable grab before doing second attempt in BA_CAN_COMMIT sequence;
- disable grab in reiser4_grab_space(), and only do that if not BA_FORCE.
reiser4: do not mess with grab_enabled; instead, use BA_FORCE.
reiser4: batch discard support: add a dummy FITRIM ioctl handler for directories.
reiser4: batch discard support: more elaborate stub.

Now with
- init work (parameters load/store)
- separate reiser4_trim_fs() function
- reservation of all possible space using new BA_ALL interface
reiser4: batch discard support: stub sleeps for 10 seconds.
hack