staging: lustre: mdc: use __FMODE_EXEC macro

FMODE_EXEC is type fmode_t but is used in operations
with integers which leads to sparse warnings:
drivers/staging/lustre/lustre/mdc/mdc_lib.c:198:21: warning: restricted fmode_t degrades to integer
drivers/staging/lustre/lustre/mdc/mdc_locks.c:300:49: warning: restricted fmode_t degrades to integer

Fix by using __FMODE_EXEC macro defined in fs.h.

Note the same warnings occurs with other fmode flags
here but they don't have a corresponding int macro.

Changes since v1:
remove ifdefs. FMODE_EXEC and __FMODE_EXEC should
always be defined in fs.h

Signed-off-by: Juston Li <juston.h.li@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>