checkpatch: Accept form feeds.

checkpatch: Accept form feeds.

CodingStyle.md says:

"Use form feeds (control+L) to divide long source files into logical
pieces.  A form feed should appear as the only character on a line."

checkpatch.py currently complains about form feed. For example, on
commit 2c06d9a927c5("ovstest: Add test-netlink-conntrack command."),
checkpatch.py returns:

    W(140): Line has non-spaces leading whitespace
    W(140): Line has trailing whitespace
    +

    W(177): Line has non-spaces leading whitespace
    W(177): Line has trailing whitespace
    +

    W(199): Line has non-spaces leading whitespace
    W(199): Line has trailing whitespace
    +

This commit suppresses the two warnings for lines with form feeds as the
only character.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>