->mnt_devname is never NULL

Not since 8f291889 "new helpers: vfs_create_mount(), fc_mount()"
back in 2018.  Get rid of the dead checks...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/20250421033509.GV2023217@ZenIV


Signed-off-by: Christian Brauner <brauner@kernel.org>