dm cache: make smq background work limit configurable

The maximum number of concurrent background work items (promotions,
demotions, writebacks) in the SMQ policy was hardcoded to 4096, with
a FIXME comment noting it should be made configurable.

This value was originally tuned down from 10240 to balance memory
overhead (~128 bytes per entry, ~512KB at 4096 entries) against I/O
parallelism. However, different workloads and cache sizes may benefit
from different limits:

- Write-heavy workloads may need more writeback concurrency
- Very large caches (10+ TB) may need more promotion slots
- Memory-constrained systems may want a lower limit

Make this configurable via the module parameter "smq_max_background_work"
(defaulting to 4096 to preserve existing behaviour). Clamp the value to
at least 1 to prevent setting 0, which would block all background work.
The parameter only affects newly created cache devices; existing caches
retain their value from creation time.

Signed-off-by: Cao Guanghui <caoguanghui@kylinos.cn>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

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