reiser4: batch discard support: actually implement the FITRIM ioctl handler.

It works in an iterative way, grabbing some fixed amount of space and allocating
blocks until grabbed space is exhausted or partition's end is reached.

Blocks are scheduled for discard by allocating them and immediately adding them
to the atom's delete set, which is read and processed at atom commit time.

After reaching the allocation stop condition, the atom is force-committed and,
if the allocation has been stopped due to grabbed space exhaustion (not due to
reaching end of partition), another iteration happens.

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>
reiser4: block_alloc: add a "min_len" parameter to reiser4_blocknr_hint to limit allocated extent length from below.

The default has been 1, that is, the first encountered free extent with any length
from 1 to requested is allocated and returned.

So, allow changing minimal extent length to allocate.

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>
reiser4: batch discard support: honor minimal extent length passed from the userspace.