Use SMgrRelation instead of SMgrRelationData * in pgaio_io_set_target_smgr()

Hi, hackers

While reading smgr.h, I noticed an inconsistent type usage in
pgaio_io_set_target_smgr(). Currently the function is declared as:

extern void pgaio_io_set_target_smgr(PgAioHandle *ioh,
SMgrRelationData *smgr,
ForkNumber forknum,
BlockNumber blocknum,
int nblocks,

However, SMgrRelation is defined as "typedef SMgrRelationData * SMgrRelation;",
and all other functions in the smgr subsystem use SMgrRelation as the parameter
type.

To keep the code consistent with the rest of the smgr API, this patch changes
the parameter from SMgrRelationData * to SMgrRelation in both the definition
and declaration.

This is purely a style/consistency cleanup with no functional change.

Thoughts? Is this change acceptable?

--
Regards,
Japin Li
ChengDu WenWu Information Technology Co., Ltd.