NFSD/blocklayout: Support multiple extents per LAYOUTGET

NFSD/blocklayout: Support multiple extents per LAYOUTGET

Allow the pNFS server to respond with multiple extents to a LAYOUTGET
request, thereby avoiding unnecessary load on the server and improving
performance for the client. The number of LAYOUTGET requests is
significantly reduced for various file access patterns, including
random and parallel writes.

Additionally, this change allows the client to request layouts with the
loga_minlength value greater than the minimum possible length of a single
extent in XFS. We use this functionality to fix a livelock in the client.

Signed-off-by: Sergey Bashirov <sergeybashirov@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>