x86/mm: Refine mmap syscall implementation

It is unnecessary to use the local variable 'error' in the mmap syscall
implementation function - just return -EINVAL directly and get rid of
the local variable altogether.

 [ bp: Massage commit message. ]

Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/20201217052648.24656-1-adrianhuang0701@gmail.com