elf, uapi: Remove struct tag 'dynamic'

Such a generic struct tag shouldn't have been exposed in a public
header.  Since it's undocumented, we can assume it's a historical
accident.  And since no software (at least on Debian) relies on this
tag, we can safely remove it.

Here are the results of a Debian Code Search[1]:

$ # packages that contain 'include [<"]linux/elf\.h[">]'
$ curl -s https://codesearch.debian.net/results/e5e7c74dfcdae609/packages.txt > include
$ # packages that contain '\bstruct dynamic\b'
$ curl -s https://codesearch.debian.net/results/b23577e099048c6a/packages.txt > struct
$ cat struct include | sort | uniq -d
chromium
hurd
linux
qemu
qt6-webengine
qtwebengine-opensource-src
$ # chromium: Seems to hold a copy of the UAPI header.  No uses of the tag.
$ # hurd:     Same thing as chromium.
$ # linux:    :)
$ # qemu:     Same thing as chromium.
$ # qt6-webengine:  Same thing as all.
$ # qtwebengine-opensource-src:  Yet another copy.

Link: https://codesearch.debian.net/ [1]
Link: https://lore.kernel.org/linux-mm/87wmxdokum.fsf@email.froward.int.ebiederm.org/T/
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Rolf Eike Beer <eb@emlix.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

include/uapi/linux/elf.h

@@ -140,7 +140,7 @@ typedef __s64	Elf64_Sxword;
 #define ELF64_ST_BIND(x)	ELF_ST_BIND(x)
 #define ELF64_ST_TYPE(x)	ELF_ST_TYPE(x)
 
-typedef struct dynamic {
+typedef struct {
   Elf32_Sword d_tag;
   union {
     Elf32_Sword	d_val;