It was introduced in "arch, x86: pmem api for ensuring durability
of persistent memory updates" in July 2015, along with the code
that really used that stuff.  Three weeks later all that code
got moved to asm/pmem.h by "pmem, x86: move x86 PMEM API to new
pmem.h header"; include, however, was left behind.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>