staging: lustre: mdt: fail FMODE_WRITE open if the client is read only

O_WRONLY/O_RDWR open on a file will get EROFS on a read only client,
but the rpc gets sent to the mdt anyway.
mdt will increase the mot_write_count of the mdt object, blocking
subsequent FMODE_EXEC open to the same file.

This patch makes sure we fail the FMODE_WRITE open with EROFS on the
client straight away without sending the rpc to mdt.

Signed-off-by: Li Dongyang <dongyang.li@anu.edu.au>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7727
Reviewed-on: http://review.whamcloud.com/18242


Reviewed-by: Ian Costello <icostello@ddn.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>