xfs: fix name/value buffer lifetime errrors

As part of kicking the tires on the new logged extended attributes
(LARP) code, I observed a crash in the xattr intent log item relogging
function.  A quick code inspection led to me noticing that the logging
code repeatedly allocates and frees space for the name and value
buffers, which is unnecessary.  Replace all that with a shared
refcounted buffer to hold the name and the value across all the log
items involved in the transaction, and as long as is necessary for the
log to process all work.
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