wrappers for ->i_mutex access

commit 5955102c upstream

parallel to mutex_{lock,unlock,trylock,is_locked,lock_nested},
inode_foo(inode) being mutex_foo(&inode->i_mutex).

Please, use those for access to ->i_mutex; over the coming cycle
->i_mutex will become rwsem, with ->lookup() done with it held
only shared.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[only the fs.h change included to make backports easier - gregkh]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>