libxlog: add a helper to initialize a xlog without clobbering the x structure

libxlog: add a helper to initialize a xlog without clobbering the x structure

xfsprogs has three copies of a code sequence to initialize an xlog
structure from a libxfs_init structure. Factor the code into a helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>