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[email protected]/
[2] https://lore.kernel.org/r/20190902122016[email protected]/
Reported-by: Christoph Hellwig <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>