pNFS: Fix extent encoding in block/scsi layout

The ext_tree_encode_commit() function may be called multiple times for
the same file, layout, and last written byte if the provided buffer is
not large enough to encode all extents in it.

The first problem is that the last written byte field must be zeroed
only on a successful call, otherwise we will lose its actual value and
get an integer overflow on the next encoding attempt.

The second problem is that we can't count and encode in one pass. The
extent state changes during encoding, so if we return -ENOSPC but have
already encoded some extents into a small buffer, they will not be
re-encoded into a new larger buffer on the next try. As a result, the
client never commits these extents to the server.

Co-developed-by: Konstantin Evtushenko <koevtushenko@yandex.com>
Signed-off-by: Konstantin Evtushenko <koevtushenko@yandex.com>
Signed-off-by: Sergey Bashirov <sergeybashirov@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250630183537.196479-3-sergeybashirov@gmail.com
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

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