staging: exfat: fix multiple definition error of `rename_file'

staging: exfat: fix multiple definition error of `rename_file'

`rename_file' was exported but not properly namespaced causing a
multiple definition error because `rename_file' is already defined in
fs/hostfs/hostfs_user.c:

ld: drivers/staging/exfat/exfat_core.o: in function `rename_file':
drivers/staging/exfat/exfat_core.c:2327: multiple definition of
`rename_file'; fs/hostfs/hostfs_user.o:fs/hostfs/hostfs_user.c:350:
first defined here
make: *** [Makefile:1077: vmlinux] Error 1

This error can be reproduced on ARCH=um by selecting:

CONFIG_EXFAT_FS=y
CONFIG_HOSTFS=y

Add a namespace prefix exfat_* to fix this error.

Reported-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Cc: stable <stable@vger.kernel.org>
Cc: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Tested-by: David Gow <davidgow@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20191204234522.42855-1-brendanhiggins@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>