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: 3fda4c61

 ("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>