xfs: scrub: use kstrdup_const() for metapath scan setups

Except 'xchk_setup_metapath_rtginode()' case, 'path' argument of
'xchk_setup_metapath_scan()' is a compile-time constant. So it may
be reasonable to use 'kstrdup_const()' / 'kree_const()' to manage
'path' field of 'struct xchk_metapath' in attempt to reuse .rodata
instance rather than making a copy. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>