grep _mntput *
namei.c: _mntput(nd->mnt);
namespace.c:void __mntput(struct vfsmount *mnt)
namespace.c:EXPORT_SYMBOL(__mntput);
CC fs/namei.o
fs/namei.c: In function `path_release_on_umount':
fs/namei.c:317: warning: implicit declaration of function `_mntput'
....
LD .tmp_vmlinux1
fs/built-in.o: In function `path_release_on_umount':
: undefined reference to `_mntput'
make: *** [.tmp_vmlinux1] Error 1
seems an underscore is missing.