xfs: remove conditional building of rt geometry validator functions

I mistakenly turned off CONFIG_XFS_RT in the Kconfig file for arm64
variant of the djwong-wtf git branch.  Unfortunately, it took me a good
hour to figure out that RT wasn't built because this is what got printed
to dmesg:

XFS (sda2): realtime geometry sanity check failed
XFS (sda2): Metadata corruption detected at xfs_sb_read_verify+0x170/0x190 [xfs], xfs_sb block 0x0

Whereas I would have expected:

XFS (sda2): Not built with CONFIG_XFS_RT
XFS (sda2): RT mount failed

The root cause of these problems is the conditional compilation of the
new functions xfs_validate_rtextents and xfs_compute_rextslog that I
introduced in the two commits listed below.  The !RT versions of these
functions return false and 0, respectively, which causes primary
superblock validation to fail, which explains the first message.

Move the two functions to other parts of libxfs that are not
conditionally defined by CONFIG_XFS_RT and remove the broken stubs so
that validation works again.

Fixes: e1429380 ("xfs: don't allow overly small or large realtime volumes")
Fixes: a6a38f30 ("xfs: make rextslog computation consistent with mkfs")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>