fix reiserfs mkdir() breakage

if directory has so many subdirectories that its link count is set
to 1 (i.e. "can't tell accurately") and reiserfs_new_inode() fails,
we shouldn't decrement the parent's link count in cleanup path;
that's what DEC_DIR_INODE_NLINK() is for.  As it is, we end up
with parent suddenly getting zero i_nlink, with very unpleasant
effects.

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