CIFS: Create routine find_writable_file to reduce redundant code

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

diff --cc fs/cifs/cifsproto.h
Simple merge

diff --cc fs/cifs/file.c
Simple merge

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