Simplify callers of diskfile.[read|write]_metadata()

As it happens, diskfile.read_metadata() and diskfile.write_metadata()
can take either an open file or a filename as their first arguments
(since xattr.[get|set]xattr() can), so we can clean up a couple places
where we were opening a file just to call read_metadata() or
write_metadata() on it. This results in 2 fewer system calls.

Example strace output:

/* read_metadata(filename) */
getxattr("/mnt/sdb1/1/node/sdb1/afile", "user.some.key", 0x0, 0) = 10
getxattr("/mnt/sdb1/1/node/sdb1/afile", "user.some.key", "some-value", 10) = 10

/* fp = open(filename); read_metadata(fp) */
open("/mnt/sdb1/1/node/sdb1/afile", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
fgetxattr(4, "user.some.key", 0x0, 0)   = 10
fgetxattr(4, "user.some.key", "some-value", 10) = 10

Change-Id: I321d8663b9e9e47b8f3ee6c21a1b65b408bb80e6
Merge "Simplify callers of diskfile.[read|write]_metadata()"