fs: Improved CONFIG_FILE_SYSTEM_MAX_FILE_NAME processing

The processing of CONFIG_FILE_SYSTEM_MAX_FILE_NAME override for
MAX_FILE_NAME has been improved to limit possibility to set it to
value that is not allowed by any of the in-tree subsystems, when
such subsystem is enabled.
When no in-tree subsystem is allowed the value is still not limited.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>