BKL: remove BKL from freevxfs

All uses of the BKL in freevxfs were the result of a pushdown into
code that doesn't really need it. As Christoph points out, this
is a read-only file system, which eliminates most of the races in
readdir/lookup.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Christoph Hellwig <hch@infradead.org>