kexec: avoid compat_alloc_user_space

kimage_alloc_init() expects a __user pointer, so compat_sys_kexec_load()
uses compat_alloc_user_space() to convert the layout and put it back onto
the user space caller stack.

Moving the user space access into the syscall handler directly actually
makes the code simpler, as the conversion for compat mode can now be done
on kernel memory.

Link: https://lkml.kernel.org/r/20210727144859.4150043-3-arnd@kernel.org
Link: https://lore.kernel.org/lkml/YPbtsU4GX6PL7%2F42@infradead.org/
Link: https://lore.kernel.org/lkml/m1y2cbzmnw.fsf@fess.ebiederm.org/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Co-developed-by: Eric Biederman <ebiederm@xmission.com>
Co-developed-by: Christoph Hellwig <hch@infradead.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Helge Deller <deller@gmx.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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