xfs: Remove all strlen in all xfs_attr_* functions for attr names.

This helps to pre-simplify the extra handling of the null terminator in
delayed operations which use memcpy rather than strlen.  Later
when we introduce parent pointers, attribute names will become binary,
so strlen will not work at all.  Removing uses of strlen now will
help reduce complexities later

Signed-off-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>