erofs: localize erofs_grab_bio()

erofs: localize erofs_grab_bio()

As Christoph pointed out [1], "erofs_grab_bio tries to
handle a bio_alloc failure, except that the function will
not actually fail due the mempool backing it."

Sorry about useless code, fix it now and
localize erofs_grab_bio [2].

[1] https://lore.kernel.org/r/20190830162812.GA10694@infradead.org/
[2] https://lore.kernel.org/r/20190902122016.GL15931@infradead.org/
Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-16-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>