ANDROID: taskstats: track fsync syscalls

This adds a counter to the taskstats extended accounting fields, which
tracks the number of times fsync is called, and then plumbs it through
to the uid_sys_stats driver.

Bug: 120442023
Change-Id: I6c138de5b2332eea70f57e098134d1d141247b3f
Signed-off-by: Jin Qian <jinqian@google.com>
[AmitP: Refactored changes to align with changes from upstream commit
        9a070004 ("sched/headers: Move CONFIG_TASK_XACCT bits from <linux/sched.h> to <linux/sched/xacct.h>")]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
[tkjos: Needed for storaged fsync accounting ("storaged --uid" and
        "storaged --task").]
[astrachan: This is modifying a userspace interface and should probably
            be reworked]
Signed-off-by: Alistair Strachan <astrachan@google.com>