erofs: allocate extra bvec pages directly instead of retrying

If non-bootstrap bvecs cannot be kept in place (very rarely), an extra
short-lived page is allocated.

Let's just allocate it immediately rather than do unnecessary -EAGAIN
return first and retry as a cleanup.  Also it's unnecessary to use
__GFP_NOFAIL here since we could gracefully fail out this case instead.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Link: https://lore.kernel.org/r/20230526201459.128169-2-hsiangkao@linux.alibaba.com