xfs: ensure rtx mask/shift are correct after growfs

xfs: ensure rtx mask/shift are correct after growfs

Source kernel commit: 86a0264ef26e90214a5bd74c72fb6e3455403bcf

When growfs sets an extent size, it doesn't updated the m_rtxblklog and
m_rtxblkmask values, which could lead to incorrect usage of them if they
were set before and can't be used for the new extent size.

Add a xfs_mount_sb_set_rextsize helper that updates the two fields, and
also use it when calculating the new RT geometry instead of disabling
the optimization there.

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>