LU-10560 mdt: remove extra headers from mdt_identity.c

LU-10560 mdt: remove extra headers from mdt_identity.c

This avoids a compile problem with 4.14 kernels:
  CC [M]  /root/lustre-release/lustre/mdt/mdt_identity.o
In file included from lustre/mdt/mdt_identity.c:49:0:
./arch/x86/include/asm/uaccess.h: In function ‘set_fs’:
  error: dereferencing pointer to incomplete type
  current->thread.addr_limit = fs;

We don't need to include <asm/uaccess.h>, also clean up
other no longer needed headers.

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: Ibcfcf3a466f7e4428994047c9d11bb557d46f9ab
Reviewed-on: https://review.whamcloud.com/31746
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>