[ipv4] Allow calculation of default subnet mask

[ipv4] Allow calculation of default subnet mask

ipv4.c calculates the default subnet mask before calling
fetch_ipv4_setting() to retrieve the configured subnet mask (if any).

However, as of commit 612f4e7 "[settings] Avoid returning
uninitialised data on error in fetch_xxx_setting()",
fetch_ipv4_setting() will zero the IP address if the setting does not
exist, rather than leaving it unaltered.

Fix by fetching the setting first and calculating the default subnet
mask only if necessary.