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 modify ctx->grab_enabled if BA_FORCE
  (thus, txnmgr_force_commit_all() will not indirectly modify it as well)
- do not re-enable grab after calling txnmgr_force_commit_all()

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>