xfs: refactor xfs_rtbitmap_blockcount

xfs: refactor xfs_rtbitmap_blockcount

Rename the existing xfs_rtbitmap_blockcount to
xfs_rtbitmap_blockcount_len and add a new xfs_rtbitmap_blockcount wrapper
around it that takes the number of extents from the mount structure.

This will simplify the move to per-rtgroup bitmaps as those will need to
pass in the number of extents per rtgroup instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>