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>