xfs: get rid of xfs_ag_resv_rmapbt_alloc

The pag in xfs_ag_resv_rmapbt_alloc() is already held when the struct
xfs_btree_cur is initialized in xfs_rmapbt_init_cursor(), so there is no
need to get pag again.

On the other hand, in xfs_rmapbt_free_block(), the similar function
xfs_ag_resv_rmapbt_free() was removed in commit 92a00544 ("xfs: get
rid of unnecessary xfs_perag_{get,put} pairs"), xfs_ag_resv_rmapbt_alloc()
was left because scrub used it, but now scrub has removed it. Therefore,
we could get rid of xfs_ag_resv_rmapbt_alloc() just like the rmap free
block, make the code cleaner.

Signed-off-by: Long Li <leo.lilong@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>