df: Improve anchor block detection

Add <last block>+1 and <last block>-1 to a list of blocks which can be the
real last recorded block on a UDF media. Sebastian Manciulea
<manciuleas@yahoo.com> claims this helps some drive + media combinations
he is able to test.

Signed-off-by: Jan Kara <jack@suse.cz>
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