m68k: Add missing mmap_read_lock() to sys_cacheflush()

commit f829b4b212a315b912cb23fd10aaf30534bb5ce9 upstream.

When the superuser flushes the entire cache, the mmap_read_lock() is not
taken, but mmap_read_unlock() is called.  Add the missing
mmap_read_lock() call.

Fixes: cd2567b6 ("m68k: call find_vma with the mmap_sem held in sys_cacheflush()")
Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20210407200032.764445-1-Liam.Howlett@Oracle.com


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
[ mmap_read_lock() open-coded using down_read() as was done prior to v5.8 ]
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>