LU-12885 llapi: use enum llapi_layout_get_flags

LU-12885 llapi: use enum llapi_layout_get_flags

For the llapi_layout_get_by_{fd,fid,path,xattr}() functions use a new
enum llapi_layout_get_flags as an argument to make it clear what flags
are passed, rather than a generic "uint32_t flags" argument.

Rename the flags to be consistent between the different functions,
instead of having separate flags for llapi_layout_get_by_xattr() and
add a LLAPI_ prefix to avoid potential namespace conflicts.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I671f1a3b96df4fa93e9529a373ceb450b13ebbe5
Reviewed-on: https://review.whamcloud.com/36524
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>