samples/subsys/mgmt/smp_svr: increase fs support stack size

Commit https://github.com/zephyrproject-rtos/zephyr/commit/0a018db0f


increases worqueue stack size for overlay-fs.conf
exclusively.
overlay-bt.conf also need to be updated as enables mcumgr FS
command set as well.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>