xfs: fix frozen file system assert in xfs_trans_alloc

Commit 83a80e95 ("xfs: decouple xfs_trans_alloc_empty from
xfs_trans_alloc") move the place of the assert for a frozen file system
after the sb_start_intwrite call that ensures it doesn't run on frozen
file systems, and thus allows to incorrect trigger it.

Fix that by moving it back to where it belongs.

Fixes: 83a80e95 ("xfs: decouple xfs_trans_alloc_empty from xfs_trans_alloc")
Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>