btrfs-progs: check: Fix false alert on generation mismatch for tree reloc tree

btrfs-progs: check: Fix false alert on generation mismatch for tree reloc tree

For fs with tree reloc tree(under balancing), lowmem mode will report
false alert like:
ERROR: extent[62914560 4096] backref generation mismatch, wanted: <=9,
have: 13

This is because lowmem mode adds a more restrict check, to ensure
generation in fs tree won't be smaller than extent tree.

In fact such assumption is not right for tree reloc tree, so remove such
check.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>