eCryptfs: fix dentry handling on create error, unlink, and inode destroy

This patch corrects some erroneous dentry handling in eCryptfs.

If there is a problem creating the lower file, then there is nothing that
the persistent lower file can do to really help us.  This patch makes a
vfs_create() failure in the lower filesystem always lead to an
unconditional do_create failure in eCryptfs.

Under certain sequences of operations, the eCryptfs dentry can remain in
the dcache after an unlink.  This patch calls d_drop() on the eCryptfs
dentry to correct this.

eCryptfs has no business calling d_delete() directly on a lower
filesystem's dentry.  This patch removes the call to d_delete() on the
lower persistent file's dentry in ecryptfs_destroy_inode().

(Thanks to David Kleikamp, Eric Sandeen, and Jeff Moyer for helping
identify and resolve this issue)

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Cc: Dave Kleikamp <shaggy@austin.ibm.com>
Cc: Eric Sandeen <sandeen@redhat.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>