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

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>