ufs: ufs_trunc_...() has exclusion with everything that might cause allocations

	Currently - on lock_ufs(), eventually - on per-inode mutex.
lock_ufs() used to be mere BKL, which is much weaker, so it needed
those rechecks.  BKL doesn't provide any exclusion once we lose CPU;
its blind replacement, OTOH, _does_.  Making that per-filesystem was
an atrocity, but at least we can simplify life here.  And yes, we
certainly need to make that sucker per-inode - these days inode.c and
truncate.c uses are needed only to protect the block pointers.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>