fs/fnctl: fix missing __user in fcntl_rw_hint()

The fcntl_rw_hint() has a missing __user annotation in
the code when assinging argp. Add this to fix the
following sparse warnings:

fs/fcntl.c:280:22: warning: incorrect type in initializer (different address spaces)
fs/fcntl.c:280:22:    expected unsigned long long [usertype] *argp
fs/fcntl.c:280:22:    got unsigned long long [noderef] [usertype] <asn:1> *
fs/fcntl.c:287:34: warning: incorrect type in argument 1 (different address spaces)
fs/fcntl.c:287:34:    expected void [noderef] <asn:1> *to
fs/fcntl.c:287:34:    got unsigned long long [usertype] *argp
fs/fcntl.c:291:40: warning: incorrect type in argument 2 (different address spaces)
fs/fcntl.c:291:40:    expected void const [noderef] <asn:1> *from
fs/fcntl.c:291:40:    got unsigned long long [usertype] *argp
fs/fcntl.c:303:34: warning: incorrect type in argument 1 (different address spaces)
fs/fcntl.c:303:34:    expected void [noderef] <asn:1> *to
fs/fcntl.c:303:34:    got unsigned long long [usertype] *argp
fs/fcntl.c:307:40: warning: incorrect type in argument 2 (different address spaces)
fs/fcntl.c:307:40:    expected void const [noderef] <asn:1> *from
fs/fcntl.c:307:40:    got unsigned long long [usertype] *argp

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>