hostfs: fix a duplicated global function name

fs/hostfs/hostfs_user.c defines do_readlink() as non-static, and so does
fs/xfs/linux-2.6/xfs_ioctl.c when CONFIG_XFS_DEBUG=y.  So rename
do_readlink() in hostfs to hostfs_do_readlink().

I think it's better if XFS guys will also rename their do_readlink(),
it's not necessary to use such a general name.

Signed-off-by: WANG Cong <wangcong@zeuux.org>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>