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: block_alloc: add BA_SOME_SPACE flag for grabbing a fixed amount of space.

This is used for FITRIM ioctl which will iteratively grab, allocate and trim disk space
bit by bit to avoid starving the rest of system.
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.
reiser4: block_alloc: add a "forward" parameter to reiser4_blocknr_hint to allocate blocks only in forward direction.
reiser4: txnmgr: free allocated but unneeded atom in atom_begin_and_assign_to_txnh().
reiser4: txnmgr: add reiser4_create_atom() which creates an empty atom without capturing any nodes.