vfs: flush and wait for io when setting the immutable flag via FSSETXATTR

When we're using FS_IOC_FSSETXATTR to set the immutable flag on a file,
we need to ensure that userspace can't continue to write the file after
the file becomes immutable.  To make that happen, we have to flush all
the dirty pagecache pages to disk to ensure that we can fail a page
fault on a mmap'd region, wait for pending directio to complete, and
hope the caller locked out any new writes by holding the inode lock.

XFS has more complex locking than other FSSETXATTR implementations so we
have to keep the checking and preparation code in different functions.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

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