mutex: Drop linkage.h from mutex.h

<linux/mutex.h> does not use nor need <linux/linkage.h>, so drop that
one header file from mutex.h.

<linux/mutex.h> is currently #included in around 1250 C source files
(oops, I didn't count other header files that #include it), making it
the 27th most-used header file.

Build tested on i386 and x86_64 * (allnoconfig, tiny.config, defconfig,
allyesconfig, and allmodconfig) and x64_64 allmodconfig + SMP=disabled.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/582b3892-4e4c-06b2-a368-5c2d439de7fc@infradead.org