vfs: fix: don't increase bio_slab_max if krealloc() fails

Without the patch, bio_slab_max, representing bio_slabs capacity, is increased before krealloc() of bio_slabs. If krealloc() fails, bio_slab_max is too high. Fix that by only updating bio_slab_max if krealloc() is successful.

Signed-off-by: Anna Leuschner <anna.m.leuschner@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>