common/options: change osd_target_transaction_size from int to uint

common/options: change osd_target_transaction_size from int to uint

Change osd_target_transaction_size from signed int to unsigned int to
match the return type of Transaction::get_num_opts() (ceph_le64).

This change:
- Eliminates compiler warnings when comparing signed/unsigned values
- Enables automatic size conversion (e.g., "4_K" → 4096) via y2c.py
  for improved administrator usability
- Maintains type consistency throughout the codebase

Signed-off-by: Kefu Chai <tchaikov@gmail.com>