Avoid tree of tree root cow when committing a clean FS

Avoid tree of tree root cow when committing a clean FS

The root node generation number code made commit_tree_root look like the
kernel code.  It forces a cow of the tree of tree roots even when
the FS hasn't changed.

This causes errors during fsck and other readonly operations.  This adds
a check to see if commit_tree_root is going to trigger writes to the
tree of tree roots, and bails if none are pending.

Signed-off-by: Chris Mason <chris.mason@oracle.com>