For execute permission on a regular files we need to check if file has
any execute bits at all, regardless of capabilites.

This check is normally performed by generic_permission() but was also
added to the case when the filesystem defines its own ->permission()
method.  In the latter case the filesystem should be responsible for
performing this check.

Move the check from inode_permission() inside filesystems which are
not calling generic_permission().

Create a helper function execute_ok() that returns true if the inode
is a directory or if any execute bits are present in i_mode.

Also fix up the following code:

 - coda control file is never executable
 - sysctl files are never executable
 - hfs_permission seems broken on MAY_EXEC, remove
 - hfsplus_permission is eqivalent to generic_permission(), remove

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>