Revert "Handle partial reads in zfs_read"

This reverts commit 59eab1093a.

The change suppressed EFAULT originating from uiomove().  The deadlock
avoidance mechanism implemented by vn_io_fault1() in the VFS handles
such errors by wiring the user pages and retrying, but this change
caused read() to return early instead.  This can result in short I/O,
causing misbehaviour in some applications, and possibly other
consequences.

Until this is resolved somehow, revert the commit.

Approved by:	mm

sys/contrib/openzfs/module/zfs/zfs_vnops.c

@@ -254,7 +254,6 @@ zfs_read(struct znode *zp, zfs_uio_t *uio, int ioflag, cred_t *cr)
 	}
 
 	ASSERT(zfs_uio_offset(uio) < zp->z_size);
-	ssize_t start_offset = zfs_uio_offset(uio);
 	ssize_t n = MIN(zfs_uio_resid(uio), zp->z_size - zfs_uio_offset(uio));
 	ssize_t start_resid = n;
 
@@ -277,13 +276,6 @@ zfs_read(struct znode *zp, zfs_uio_t *uio, int ioflag, cred_t *cr)
 			/* convert checksum errors into IO errors */
 			if (error == ECKSUM)
 				error = SET_ERROR(EIO);
-			/*
-			 * if we actually read some bytes, bubbling EFAULT
-			 * up to become EAGAIN isn't what we want here.
-			 */
-			if (error == EFAULT &&
-			    (zfs_uio_offset(uio) - start_offset) != 0)
-				error = 0;
 			break;
 		}