vfs_cluster.c: Do not propagate VOP_BMAP errors to the caller

The code that makes this VOP_BMAP call tries to perform a read-ahead I/O
operation. Failing to do that for any reason isn't fatal for cluster_read(),
because we still can return some data to the caller. This change is consistent
with other places within cluster_read(), where error returned by VOP_BMAP is
not returned to the caller - see the if (nblks > 1) block above the changed
lines and if (reqbp) at the end of the function.

PR: 264196
Approved by: markj, kib
Differential Revision: https://reviews.freebsd.org/D51254