new helper: is_root_inode()

replace open-coded instances

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/coda/coda_linux.c
Simple merge

diff --cc fs/coda/coda_linux.h
Simple merge

diff --cc fs/coda/dir.c
Simple merge

diff --cc fs/ncpfs/dir.c
Simple merge

diff --cc include/linux/fs.h
Simple merge