udf: Remove useless check in udf_adinicb_write_begin()

As Al properly points out, len is guaranteed to be smaller than
PAGE_SIZE when we reach udf_adinicb_write_begin() as otherwise we would
have converted the file to the normal format.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Jan Kara <jack@suse.cz>