UPSTREAM: fsverity: don't drop pagecache at end of FS_IOC_ENABLE_VERITY

The full pagecache drop at the end of FS_IOC_ENABLE_VERITY is causing
performance problems and is hindering adoption of fsverity.  It was
intended to solve a race condition where unverified pages might be left
in the pagecache.  But actually it doesn't solve it fully.

Since the incomplete solution for this race condition has too much
performance impact for it to be worth it, let's remove it for now.

Fixes: 3fda4c617e84 ("fs-verity: implement FS_IOC_ENABLE_VERITY ioctl")
Cc: stable@vger.kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Link: https://lore.kernel.org/r/20230314235332.50270-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>

Bug: 273320626
(cherry picked from commit a075bacde257f755bea0e53400c9f1cdd1b8e8e6)
Change-Id: I28dacf122bba5ac816f9b748dcbaa82dc1072fed
Signed-off-by: Eric Biggers <ebiggers@google.com>