scsi: sg: protect against races between mmap() and SG_SET_RESERVED_SIZE

commit 6a8dadcca81fceff9976e8828cceb072873b7bd5 upstream.

Take f_mutex around mmap() processing to protect against races with the
SG_SET_RESERVED_SIZE ioctl.  Ensure the reserve buffer length remains
consistent during the mapping operation, and set the "mmap called" flag
to prevent further changes to the reserved buffer size as an atomic
operation with the mapping.

[mkp: fixed whitespace]

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>