Add llite file flag to allow skipping the locking for file read/write ops.

Add llite file flag to allow skipping the locking for file read/write ops.

The ioctl LL_IOC_SETFLAGS and LL_IOC_CLRFLAGS can be used to set and clear
the LL_FILE_IGNORE_LOCK flag on a file, and LL_IOC_GETFLAGS can be used to
retrieve the current flags value.  These values are defined in lustre_lite.h.

Also add a small optimization - we do not update the file atime on reads if
it would not change the actual atime value (i.e. do not update this field
more often than once per second).  We also avoid updating the atime on R/O
mounted filesystems or if the "noatime" mount option is used.