fs: factor out common code in fget() and fget_raw()

Apart from FMODE_PATH check fget() and fget_raw() are identical,
shift the code into the new simple helper, __fget(fd, mask). Saves
160 bytes.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>