LU-15396 osd: include linux/file.h

LU-15396 osd: include linux/file.h

in some 4.x kernels we need to include linux/file.h to have
alloc_file() defined.

Fixes: b0f150eba4 ("LU-13783 osd-ldiskfs: use alloc_file_pseudo to create fake files")
Test-Parameters: trivial
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I279945f70578030bf581fa2afc0ca7b4dfa83653
Reviewed-on: https://review.whamcloud.com/45923
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>