e2fsck: exit from preenhalt if IO errors were encountered

e2fsck: exit from preenhalt if IO errors were encountered

If a block device is read-only, e2fsck -p gets into an infinite loop
trying to preenhalt, closing and flushing the fs, which tries to flush
the cache, which gets a write error and calls preenhalt which tries to
close and flush the fs ... ad infinitum.

Per Ted's suggestion just flag the ctx as "exiting" and short-circuit
the infinite loop.

Tested by running e2fsck -p on a block device set read-only by BLKROSET.

Thanks to Vlado Potisk for reporting this.

Addresses-Red-Hat-Bugzilla: #465679

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>