ext4: move flush_completed_IO to fs/ext4/fsync.c and make it static

Fix a namespace leak by moving the function to the file where it is
used and making it static.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --cc fs/ext4/ext4.h
Simple merge

diff --cc fs/ext4/fsync.c
Simple merge

diff --cc fs/ext4/inode.c
Simple merge