ext4: Add support for FIDTRIM, a best-effort ioctl for deep discard trim

* What
This provides an interface for issuing an FITRIM which uses the
secure discard instead of just a discard.
Only the eMMC command is "secure", and not how the FS uses it:
due to the fact that the FS might reassign a region somewhere else,
the original deleted data will not be affected by the "trim" which only
handles un-used regions.
So we'll just call it "deep discard", and note that this is a
"best effort" cleanup.

* Why
Once in a while, We want to be able to cleanup most of the unused blocks
after erasing a bunch of files.
We don't want to constantly secure-discard via a mount option.

From an eMMC spec perspective, it tells the device to really get rid of
all the data for the specified blocks and not just put them back into the
pool of free ones (unlike the normal TRIM). The eMMC spec says the
secure trim handling must make sure the data (and metadata) is not available
anymore. A simple TRIM doesn't clear the data, it just puts blocks in the
free pool.
JEDEC Standard No. 84-A441
  7.6.9 Secure Erase
  7.6.10 Secure Trim

From an FS perspective, it is acceptable to leave some data behind.
 - directory entries related to deleted files
 - databases entries related to deleted files
 - small-file data stored in inode extents
 - blocks held by the FS waiting to be re-used (mitigated by sync).
 - blocks reassigned by the FS prior to FIDTRIM.

Change-Id: I676a1404a80130d93930c84898360f2e6fb2f81e
Signed-off-by: Geremy Condra <gcondra@google.com>
Signed-off-by: JP Abgrall <jpa@google.com>
sync: Fix a race condition between release_obj and print_obj

Before this change, a timeline would only be removed from the timeline
list *after* the sync driver had its release_obj() called. However, the
driver's release_obj() may free resources needed by print_obj().

Although the timeline list is locked when print_obj() is called, it is
not locked when release_obj() is called. If one CPU was in print_obj()
when another was in release_obj(), the print_obj() may make unsafe
accesses.

It is not actually necessary to hold the timeline list lock when calling
release_obj() if the call is made after the timeline is unlinked from
the list, since there is no possibility another thread could be in --
or enter -- print_obj() for that timeline.

This change moves the release_obj() call to after the timeline is
unlinked, preventing the above race from occurring.

Change-Id: I4cb2a07ebd79e1d553d17af8ff5bb49f89250f23
Signed-off-by: Alistair Strachan <alistair.strachan@imgtec.com>