do_last(): fix missing checks for LAST_BIND case

/proc/self/cwd with O_CREAT should fail with EISDIR.  /proc/self/exe, OTOH,
should fail with ENOTDIR when opened with O_DIRECTORY.

Change-Id: I01c85a6a3894c6854c604f192f221175edc19867
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>