fs/fcb: improve error handling in fcb_offset_last_n

Fix for not served error code in fcb_offset_last_n()
implementation.
Issue was reported by static code analyze scan.
Looks like the function might have failed silently
if aggressive access to the fcb instance
from another thread have been occurring while the function
have been processing the fcb storage.

fixes #20512

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