ufs rename: ensure that the result of ufs_checkpath() is stable

ufs_rename() calls ufs_checkpath() to ensure that the target directory
is not a child of the source. If not, rename would create a loop.
For instance:
source->X1->X2->target
and if source moved under target, we get corrupted filesystem.
Suppose that we initially have
source->X1 .... and X2->target
where X1 is not on path from root to X2. Then ufs_checkpath() accepts
the inodes, but there is nothing preventing parallel rename of X2 to become
under X1, after checkpath finished.

Ensure stability of ufs_checkpath() result by taking a per-mount sx in
ufs_rename right before ufs_checkpath() and till the end.

Reviewed by: chs, mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks