vfs: syscall: Add fsmount() to create a mount for a superblock

Provide a system call by which a filesystem opened with fsopen() and
configured by a series of fsconfig() calls can have a detached mount object
created for it.  This mount object can then be attached to the VFS mount
hierarchy using move_mount() by passing the returned file descriptor as the
from directory fd.

The system call looks like:

	int mfd = fsmount(int fsfd, unsigned int flags,
			  unsigned int attr_flags);

where fsfd is the file descriptor returned by fsopen().  flags can be 0 or
FSMOUNT_CLOEXEC.  attr_flags is a bitwise-OR of the following flags:

	MOUNT_ATTR_RDONLY	Mount read-only
	MOUNT_ATTR_NOSUID	Ignore suid and sgid bits
	MOUNT_ATTR_NODEV	Disallow access to device special files
	MOUNT_ATTR_NOEXEC	Disallow program execution
	MOUNT_ATTR__ATIME	Setting on how atime should be updated
	MOUNT_ATTR_RELATIME	- Update atime relative to mtime/ctime
	MOUNT_ATTR_NOATIME	- Do not update access times
	MOUNT_ATTR_STRICTATIME	- Always perform atime updates
	MOUNT_ATTR_NODIRATIME	Do not update directory access times

In the event that fsmount() fails, it may be possible to get an error
message by calling read() on fsfd.  If no message is available, ENODATA
will be reported.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-api@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/x86/entry/syscalls/syscall_64.tbl

@@ -347,6 +347,7 @@
 336	common	move_mount		__x64_sys_move_mount
 337	common	fsopen			__x64_sys_fsopen
 338	common	fsconfig		__x64_sys_fsconfig
+339	common	fsmount			__x64_sys_fsmount
 # don't use numbers 387 through 423, add new calls after the last
 # 'common' entry
 424	common	pidfd_send_signal	__x64_sys_pidfd_send_signal